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Old 02-19-2020, 04:47 AM   #27 (permalink)
serialk11r
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Quote:
Originally Posted by EcoCivic View Post
I know that there are 5W and 0W racing oils out there. As for standard oils, some cars now use 0W16 oil. I wonder how much the oil viscosity actually affects efficiency within reason.

Realistically the fuel efficiency difference between, say, 5w20 and 5w30 oil is probably under 1 percent from the research I did, under most conditions anyways.
I think the answer is it affects efficiency quite a bit, but it's got a bit of a negative feedback loop built in.

Thin oil has less viscous drag, which means it heats up less at the same temperature and load. Thick oil has more, so the friction heats it up and brings up the temperature, lowering the viscosity. If you have very effective oil cooling and a large sump with thick oil, you're definitely losing fuel efficiency in a noticeable way.

Some engines are built with bigger bearing tolerances so they need the high viscosity. As far as I can tell, it's better if you don't need to rely on this effect, e.g. F1 engines which run a 0 weight or something (but they also need to be pre-warmed). Putting thicker oil in an engine built for thinner oil is only useful if you can't keep the oil temp under control in severe load conditions, e.g. pouring some 50 weight in at a track day if your oil temps are like 300F.
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