I've switched back and forth and even on a small 4 banger mercedes engine which got pretty good gas mileage (so the delta will be more noticeable) I've never really perceived any gas mileage change. It would be interesting to see the difference between 20w50 and 10w30 oil on a 50 mpg engine. That's where you'd see the most difference. I've never had an engine like that.
On a big v-8 you'll have the most surface area affected, but you suck so much gas anyway you can't see the difference much anyway.
I've mainly ran the heavier oil to get the increased lower oil weight and it's what the dealers use locally when I did it. When in doubt, I'd defer to what the local [good] dealers use. They know what they are doing (usually). That's a AAA dealer that is, as I don't deal with anything less (and I don't have to). If you're going to pay, pay a little more and get the best, it saves in the long run.
The one area though, and I've never run into it myself on an auto, that might be a bit of a caveat is the increased oil pressure when run at high speed and the engine is still cold. The only time I ever ran into an issue with oil too thick was when I ran straight 60 weight oil in an old yamaha motorcycle. Waaaaaaay too much oil pressure! It was kind of dumb but I was dumb then too.......
Severe winter weather aside. One time during sub 20 below weather I got caught with a Triumph TR7 and I think I had the standard 20w50 oil in it. The oil was so thick that even when I had my mother tow me around in an iced over k-mart parking lot to get the engine to turn over and start it took me a full loop and a half around the *whole* parking lot to get the wheels to turn over enough to get the engine to turn over to bump start it. But that was on almost pure ice too. Nasty, nasty day. That's when a LOT of people found that their car wouldn't start.
After that I took the battery out of my Olds 442 (loved the big v-8 in that thing!) and put it in my TR7. Never had an issue with turning the engine over after that. I had a lot of battery power then!
But *that* is when you need 5w oil. Anything above that is fine with 10w. Unless it's synthetic... To me it's crazy to use 5w oil when the coldest you'll see is 50 degrees. Again, unless it's synthetic..... since then it doesn't really matter. You can easily run that down to 0w oil if you want and you'll still be protected.
Keeping your oil clean is the single most important thing in using the "right" oil. There is lots of room for negotiation on viscocity.
Alan
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Originally Posted by KLS
15W-50 or 20W-50 is for engines built with loose clearances or engines with clearances worn well beyond what they should be. In a tight engine I think you'll find that you'll get higher internal friction and slightly worse gas mileage with no benefit.
Ken
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..not saying it will happen but why chance it if u dont have to?
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