The company I run makes billet connecting rods for Jr. Dragster and Racing Kart engines. My #1 goal is "no broken rods." In order to move toward that goal I have had to learn a lot about oil since the improper application of oil is the 2nd biggest culprit in breaking rods. (Improper torque is #1)
Anyway, I have had the unfortunate opportunity to study oil related failure in more racing engines than most folks. The after effect can tell you a lot about what not to do, as well as what works. I have also been able to pick the brains of oil application engineers from both Mobil Oil and Castrol.
95% of the rods my company makes go in engines that do not have oil pumps, or filters. Oil gets to where it needs to be by splash. I like to call it "oil by chance." One thing is certain in the small engines, oil viscosity is important. Too thick, and it won't go where it is needed. Too thin and it will not provide enough cushion to keep metal parts from banging together.
If you go to any used car dealership, you are sure to find some motor honey or 50 wt oil somewhere in the back shed. They put this thick oil in to take up oil clearances in older, worn engines to keep it from ticking and rattling. The oil is providing more cushion.
Most new cars and trucks today recommend 5w30, or even 5w15 oil as compared to cars of the 80's that usually used 10w30. The higher the number, the thicker the oil's viscosity. All oil, unless mixed with special polymers, will get thinner as the temperature goes up. Multi-grade oils like 10w30 or 5w30 actually have a thermostatically controlled thickener that counteracts the oil's natural thinning as temperatures increase. The first number is more of a cold viscosity, and the second number is a running viscosity. Although the first number does effect the second number, so a 10w30 is thicker at running temp than a 5w30.
But why would cars of today run thinner oil? Because they are simply machined better than they used to be and are made of better materials. Clearances between bearing surfaces are tighter and have closer tolerances. It is hard for the oil pump to effectively push high viscosity oil into these tight areas, and since there is very little "wiggle room," there is not as much need for cushioning.
Think of the oil pump as a runner. If he is running on the beach it is easy. If he is running knee deep in the water, it is not. And it is almost imposible to run knee deep in mud. That is how your oil pump sees oil. It requires greater energy to push thick oil. And to a lesser extent, the volume of oil in the crankcase also creates drag. Think of it like running in ankle deep water vs waist deep water, so don't put more oil in than what is needed.
Now racing engines create some seriously different environments than what is seen in your average passenger car. Temperatures are higher, pressures are greater, and the surface footage of bearings is greater due to the higher RPM. So what is right for the family car is not sufficient for a racing engine. Especially like the one we use, which is air cooled.
The point is that our engines will require a higher viscosity because of the added demand.
A quick note about synthetic oils. The number one reason why a synthetic oil is better is because it can stand higher temperatures before breaking down (burning). Once an oil starts to break down, it only gets worse faster. You can put non-synthetic in your engine if you like, but I'm going with a 100% synthetic for the added protection and lower operating temperatures it yields. As long as you have a good filter, you can leave it in your engine twice as long as conventional oils so you will likely save money.
Back to viscosity. The engine we use is a motorcycle engine that originally used 5w30 just like every other Japanese bike of its day. 10w30 would be ok in hotter climates. We are asking so much from these engines that we need more than that. So most folks go to a 20w50. To me, that is akin to running knee deep in mud when we need to be knee deep in water. I prefer a 15w40, but you can't find that except for diesel engines. So I mix 10w30 and 20w50 half and half.
Remember that we have wet clutches in Legends, so DO NOT add slickeners like teflon to your oil, or your clutch will not love you.
That is all my opinion only. I honestly would like to discuss any different opinions. If you know an "oil guru," send him here. I don't mind being corrected as long as it is explained.
