GMC Western States

 Tech Center Number 34 - August 15, 2001

Emery Stora

From our Technical Vice President

I hope that many of you are coming to our Mariposa Rally on September 17 to 22. I am looking forward to again seeing old friends and making some new ones.
I am still waiting for some suggestions for future Tech Center articles. I have only had one request in the last six months and that is for information on building cornering lights. I will cover that in the next bulletin. Meanwhile, I will cover some topics that are of interest to me and which I hope you also will find of value.

It is very important to remember to periodically grease you chassis and replace the motor oil and filter. Also don't forget to repack the front wheel bearings and change the transmission oil and filter periodically. The final drive also needs periodic oil changing.

There are normally a total of 17 Zerk fittings for grease. However my original steering relay lever was changed last year and I now have an 18th grease fitting on the rear top of the lever where a ball joint is attached. GM's recommended schedule is every 3,000 miles or 3 months (whichever comes first). Most GMCers recommend that the rear suspension fittings be greased every 1,000 miles and that, to do it right, the side should be jacked up and the air bags deflated so that the wheel can be moved up and down while greasing. This insures getting the grease all the way through and around the pins. You can use a shovel or a pry bar under the elevated tire to move it while greasing. (Probably best to have a helper while doing this.) If on a trip at least grease the fittings at 1,000 miles with the wheels on the ground, and then when you get back from the trip, lift the wheels and do it right.

The change period for engine oil is also at 3,000 miles or 3 months. The oil filter should be changed every second oil change. This is also a good time to lubricate the transmission shift linkage, brake pedal spring, parking brake cable guides and linkage.

The automatic transmission fluid and filter should be changed every 12,000 miles or 12 months, whichever comes first. GM also indicates that if the vehicle is driven under unusual conditions such as heavy traffic (stop and go) during hot weather or where the engine idles for long periods, it should be changed at 6,000 mile intervals.

The rear wheel bearings and the front wheel bearings should be cleaned and repacked every 24,000 miles. Some recommend replacing the front wheel bearings every time instead of repacking them, but at our March 2001 Rally the Timken representative said that if the bearings do not have any scores or pits and have no blue discoloration, they can be reused. Brown staining from the grease is all right, but blue indicates overheating of the bearings.

More and more people are using 100% synthetic oils and greases such as Mobil 1 and Amsoil. Apparently the oil change period can be quite a bit longer with synthetics, as they don't break down in viscosity and don't degrade with heat as easily as regular oils.

While on the topic of lubrication, I thought it might be a good time to review some information about oils in general. Choosing the best motor oil is a topic that comes up frequently in discussions between GMCers. Oil companies provide data on their oils most often referred to as "typical inspection data". This is an average of the actual physical and a few chemical properties of their oils. This information is available to the public through their distributors or by writing or calling the company directly. Below is a list of some popular premium oils so you can make a comparison. If your favorite oil is not on the list, get the data from the distributor and use what I have as a database. The information I am presenting is from data from the manufacturers of oils and from the Internet including information compiled by Tom Coradeschi and Ed Hackett.

We are going to look at six of the most important properties of a motor oil readily available to the public: viscosity, viscosity index (VI), flash point, pour point, % sulfated ash, and % zinc.

Viscosity is a measure of the "thickness" or "flowability" of oil. It is a property of oil to develop and maintain a certain amount of shearing stress dependent on flow, and then to offer continued resistance to flow. Thicker oils generally have a higher viscosity, and thinner oils a lower viscosity. This is the most important property for an engine. Oil with too low a viscosity can shear and lose film strength at high temperatures. Oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm. The weight given on a can of oil is really just an arbitrary number assigned by the S.A.E. (Society of Automotive Engineers). However, these numbers correspond to "real" viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, and 50 by the S.A.E. The W designation after the weight number means the oil is suitable for winter use. So, a 10W weight oil measured at 0 deg. C would be of much lower viscosity than a 10 weight oil (without the W designation) also measured at 0 deg. C.

The following chart shows the relationship of "real" viscosity (numbers 2 through 42) to their S.A.E. assigned numbers at 100 deg. C. The relationship of gear oils to engine oils is also shown. Many people are not aware that gear oils and motor oils are rated differently for viscosity. Looking at this chart you can see that 90 "weight" gear oil has about the same viscosity as 40 to 50 "weight" motor oil (at 100 deg. C)

 SAE Gear Viscosity Number






SAE Crankcase Viscosity Number






Real Viscosity in centistokes @ 100 degrees C.
2 4 6 8 10 1 2 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

Multi viscosity oils work like this: Polymers, called VI (viscosity index) Improvers, are added to a light base oil (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. I often suggest the image of trying to pour a can of peas and a can of spaghetti. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

Multi viscosity oils are one of the great improvements in oils, but they should be chosen wisely. Always use a multi grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter base your decision on the lowest temperature you will encounter and in the summer the highest temperature you expect. The polymers can shear and burn and form deposits that can cause ring sticking and other problems. 10W-40 and 5W-30 require a lot of polymers (synthetics excluded) to achieve that range. This has caused problems in diesel engines, but fewer polymers are better for all engines. The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the higher polymer content. It is the oil that lubricates not the additives. Oils that can do their job with the fewest additives are the best. One of the problems with multi viscosity oils is that, over time, the polymers can shear and your oil will thin down. This is another reason to change your oil at regular intervals.

Very few manufactures recommend 10W-40 any more, and some threaten to void warranties if it is used. 20W-50 has the same 30 point spread, but because it starts with a heavier base, it requires less VI improvers (polymers) to do the job. AMSOIL can formulate their 10W-30 and 15W-40 with no viscosity index improvers but uses some in their 10W-40 and 5W-30. Mobil 1 uses no viscosity improvers in their 5W-30 and in their 10W-30. I believe they use a small amount in the 15W-50 oil. Follow your manufacturer's recommendations as to which weights are appropriate for your vehicle.

Viscosity Index (VI)
Viscosity Index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change; lower numbers indicate a relatively large change. The higher the number the better. This is one major property of an oil that keeps your bearings happy. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.

Flash Point
Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. The flash point can be an indicator of the quality of the base stock used. The higher the flash point the better. 400 F is the minimum to prevent possible high consumption. Flash point is in degrees F.

Pour Point
Pour point is 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. Some manufacturers give a borderline pumping temperature. This is the temperature at which the oil will pump and maintain adequate oil pressure. This was not given by a lot of the manufacturers, but seems to be about 20 degrees F above the pour point. The lower the pour point the better. Pour point is in deg. F.

Sulfated ash
Sulfated ash is how much solid material is left when the oil burns. A high ash content will tend to form more sludge and deposits in the engine. Low ash content also seems to promote long valve life. Look for oils with a low ash content.

Zinc is the amount of zinc used as an extreme pressure anti-wear additive. The zinc is only used when there is actual metal-to-metal contact in the engine. Hopefully the oil will do its job and this will rarely occur, but if it does, the zinc compounds react with the metal to prevent scuffing and wear. A level of .11% is enough to protect an automobile engine for the extended oil drain interval under normal use. More doesn't give you better protection; it gives you longer protection if the rate of metal-to-metal contact is abnormally high. But note that high zinc content can lead to deposit formation and plug fouling.

The Data
Listed alphabetically. --- indicates the data was not available (or does not apply to synthetics).

 Brand  VI  Flash  Pour  %ash  %zinc
 AMSOIL  157  507  -44  ---  ---
 Castrol GTX  122  440  -15  .85  .12
 Exxon High Performance  119  419  -13  .70  .11
 Havoline Formula 3  125  465  -30  1.0  ---
 Kendall GT-1  129  390  -25  1.0  .16
 Pennzoil GT Perf.  120  460  -10  .90  ---
 Quaker State Dlx  155  430  -25  .90  ---
 Shell Truck Guard  130  450  -15  1.0  .15
 Unocal  121  432  -11  .74  .12
 Valvoline All Climate  125  430  -10  1.0  .11
 Valvoline Race  140  425  -10  1.2  .20
 AMSOIL  124  500  -49  ---  ---
 Castrol Multi-Grade  110  440  -15  .85  .12
 Quaker State  121  415  -15  .90  ---
 Chevron  204  415  -18  .96  .11
 Mobil 1  170  470  -55  ---  ---
 Castrol Syntec  180  437  -45  1.2  .10
 Quaker State Synquest  173  457  -76  ---  ---
 Pennzoil Performax  176  ---  -69  ---  ---
 AMSOIL  164  462  -49  ---  ---
 Castrol  134  415  -15  1.3  .14
 Chevron Delo 400  136  421  -27   1.0   ---
 Exxon XD3 Extra  135  399  -11  .95  .13
 Kendall GT-1  135  410  -25  1.0  .16
 Shell Rotella w/XLA  146  410  -25  1.0  .13
 Valvoline Turbo  140  420  -10  .99  .13
 AMSOIL  162  520  -76  ---  ---
 Castrol GTX  140  415  -33  .85  .12
 Chevron Supreme  150  401  -26  .96  .11
 Exxon Superflo Hi Perf  135  392  -22  .70  .11
 Kendall GT-1  139  390  -25  1.0  .16
 Mobil 1  160  450  -65  ---  ---
 Pennzoil PLZ Turbo  140  410 -27 1.0  ---
 Quaker State  156  410  -30  .90  ---
 Shell Truck Guard  155  405  -35  1.0  .15
 Unocal Super  153  428  -33  .92  .12
 Valvoline All Climate  130  410  -26  1.0  .11
 Valvoline Synthetic  140  450  -40  <1.5  .12
 AMSOIL  186  464  -76  ---  ---
 Castrol GTX  156  400  -35  .80  .12
 Chevron Supreme  202  354  -46  .96  .11
 Chevron Supreme Synt.  165  446  -72  1.1  .12
 Exxon Superflow HP  148  392  -22  .70  .11
 Havoline Formula 3  158  420  -40  1.0  ---
 Mobil 1  165  445  -65  ---  ---
 Quaker State  165  405  -35  .90  ---
 Shell Fire and Ice  167  405  -35  .90  .12
 Unocal  151  414  -33  .81  .12
 Valvoline All Climate  135  405  -40  1.0  .11
 Valvoline Synthetic  160  435  -40  <1.5  .12
10W-40 is not listed as today many manufacturers indicate that you should not use it.

All of the oils above meet current SG/CD ratings and all vehicle manufactures' warranty requirements in the proper viscosity. All are "good enough", but those with the better numbers exceed the auto manufacturers' minimum specs.

The synthetics offer the only truly significant differences due to their superior high temperature oxidation resistance, high film strength, very low tendency to form deposits, stable viscosity base, and low temperature flow characteristics. Synthetics are superior lubricants compared to traditional petroleum oils. You will have to decide if their higher cost is justified.

The extended oil drain intervals given by the vehicle manufacturers (typically 7500 miles) and synthetic oil companies (up to 25,000 miles) are for what is called normal service. Normal service is defined as the engine at normal operating temperature, at highway speeds, and in a dust free environment. Stop and go, city driving, trips of less than 10 miles, or extreme heat or cold puts the oil change interval into the severe service category, which is 3000 miles for most vehicles. GM considered the GMC service as being extreme so they recommended 3000 mile changes. Synthetics can be run two to three times the mileage of petroleum oils with no problems. They do not react to combustion and combustion by-products to the extent that regular oils do. The longer possible drain intervals help take the bite out of the higher cost of the synthetics. The recommended drain intervals of the manufacturer are set for petroleum oils, and the manufacturers make no official allowance for the use of synthetics.

The numbers above are not, by any means, all there is to determining what makes top quality oil. The exact base stock used, the type, quality, and quantity of additives used are very important. The given data combined with the manufacturer's claims, your personal experience, and the reputation of the oil among others who use it should help you make an informed choice.

In my personal opinion, oil additives should not be used. The oil companies have gone to great lengths to develop an additive package that meets the vehicle's requirements. Among other things they add VI improvers, antifoam agents, corrosion inhibitors, extreme pressure additives, detergents, and dispersant agents. Some of these additives are synergistic, that is the effect of two additives together is greater than the effect of each acting separately. If you add anything to the oil you may upset this balance and prevent the oil from performing to specification.

When you really think about it, the major oil companies are some of the richest, most powerful and aggressive corporations in the world. They own multi-million dollar research facilities manned by some of the best chemists and engineers money can hire. It is probably safe to say that any one of them has the capabilities and resources at hand in marketing, distribution, advertising, research and product development equal to 100 times that of any of the independent additive companies. It therefore stands to reason that if any of these additive products were actually capable of improving the capabilities of engine lubricants, the major oil companies would have been able to determine that and to find some way to cash in on it. Yet of all the oil additives I have seen, none carried the name or endorsement of any of the major oil producers. In fact, I have seen many directives from oil companies warning not to add any additives to their products.

 This material is based on my personal experience and the personal experience of other club members. It is our viewpoint and does not represent authorized data pertaining to the GMC Motorhome. It is the responsibility of the readers to make their own judgment as to the validity of this material in relation to any repairs and/or modifications to their own vehicles.

 3128 Vista Sandia, Santa Fe, NM  87501-8526

 (505) 989-8157

 Please send your comments and ideas for the Tech Center to:
Emery Stora, Technical Vice President,
3128 Vista Sandia, Santa Fe, NM  87501-8526
Phone: (505) 989-8157



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