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Can you use car oil in a motorcycle?

Can you use car oil in a motorcycle?


Car vs. Motorcycle Oils

We’ve been told over and over that we shouldn’t use auto oils in our motorcycles. The usual reason offered is that our bikes are air-cooled and car oils aren’t designed for that use. But there have been air-cooled autos, and they use car oil! So what are the differences between car and bike oils? You can ask the oil makers and you’ll come away with the feeling you’ve been talking with a snake oil medicine show dude. They will not give you a straight answer.

mobil 1 oil for motorcyclesIn theory the base oil stock should be as pure as possible in either case. While there might be some differences, it would seem the oil companies would then tout those differences. They don’t. Generally they just point to the additive packages as being the difference. So what are the differences in the packages that makes one oil suitable for an air-cooled engine and another for water-cooled motors? I’ll pick a few commonly used oils and list some additive levels. Obviously I’m not going to cover every oil someone might use, but you’ll see a pattern. All are 20w-50, except the Mobil 1 15w-50 car oil.

M1 V-Twin Moly-91 Bo-255 Ca-2648 Mg-13 Ph-1683 Zn-1823
M1 15w-50 car Moly-79 Bo-281 Ca-2937 Mg-17 Ph-1223 Zn-1376
HD Syn 3 Moly-1 Bo-271 Ca-1342 Mg-740 Ph-997 Zn-1068
Castrol GTX car Moly-? Bo-0 Ca-1163 Mg-259 Ph-928 Zn-1133
Havoline car Moly-230 Bo-170 Ca-2461 Mg-66 Ph-978 Zn-1093

As a reminder:
Anti-wear – Zinc, phosphorus, molybdenum, manganese, potassium, and sodium.
Acid neutralizers – Boron, calcium, and magnesium.
Friction modifier – Moly.

See a pattern? If you do, you’ve got a better eye than mine. The pattern I see is there is no pattern. Some bike oils appear better than car oils. Some car oils are better than other bike oils. I see an $8/quart bike oil that has an excellent anti-wear package, almost to the over-kill point. I see a $5/quart car oil that appears much better than the MoCo $8/quart oil. Finally, in this short series, a famous car (dino) oil doesn’t appear to look as good as the other examples. But another car (dino) oil, at about $2/quart, shows to be a superb oil.

Look at the examples, balance the differences in additive levels with price, throw in how often you change your oil, and draw your own conclusions.

Synthetics vs. Dino Oils
We know that synthetic oils cost significantly more than regular oils. Is the increased cost worth it? It seems to boil down to, ”That depends!!”

Synthetic oil withstands heat better than dino oils. A temperature that will literally cook (oxidize) a regular oil will have no effect on a good synthetic. If you regularly ride in congested city traffic, participate in parades, or ride in an area where the temperatures are often well above 90º F, then a synthetic may give you years more use of your engine. If you ride a Twin Cam HD, remember your oil temperatures can be in the 250º F range. This is very close to the upper limit of dino oil, and you are going to experience increased oil oxidation, more shear, and the resultant decrease in viscosity. If you change your oil at intervals of more than 3000 miles, the inherent ability of synthetics to retain their viscosity better, will likely give you better engine longevity. If you change oil at 2000 to 3000 mile intervals, don’t experience extreme temperatures in your area, and seldom ”stress” your engine, then a synthetic may be more than you need.

Engine Break-in
One of the old wives’ tales was you couldn’t break in a motor with synthetic oil. It was said the rings wouldn’t seat for one thing. It’s been proved otherwise, but you’ll still hear it every now and again.

If you go to several oil manufacturer’s sites you’ll see their responses to questions concerning break-in with synthetics. One of the sites lists about ten vehicles that come with factory fills of synthetic oil. Doesn’t seem to cause a break-in problem with these vehicles, so why worry about it?