No flames just an honest question & answer, could a lighter weight improve MPG. LOL Maybe M1 75w-90 "not sure if the last # matters much"? It gets cold here and I'm sure its like honey during the winter.

No. You would never notice any difference between to two oils. Put 30 wt in it and I doubt there would be any difference. Fill a coffee cup with 90 wt and another with 30 wt oil and set them outside in January. Both will be pretty stiff come morning. Out here in the blazing sun we should use 225/300 Multigrade in the summer. Starting this week I quit using 20-50 motor oil much to my chagrin. I have used 20-50 on everything for 20 years. I am going through withdrawal right now.

I still use i20W50 when any of my vehicles turn over 100,000 miles. That is typically the heaviest grade the manufacturer lists and something my Father taught me to do with high mileage vehicles. Here in the warm California climate it is seldom out of spec, temperature-wise.

What's the reasoning behind this? Did he say why?

I just switched my DD beater ('85 bmw 325e with 180k miles) from 20W-50 dino to 15W-50 synthetic. All I've ever run in it is 20W-50, but that's what the owners manual calls for in temps of 30-110 F. In addition to a tick I suspect was a valve sticking slightly, I noted some sludging on the tops of the rocker arms through the oil fill hole and said what the heck, time for a change from the dino. I know you're not supposed to switch over to synthetic at that high mileage, but it's a beater and I figured what the hell.


Long story short, the thinner synthetic (along with 1Q of Rislone) killed the ticking and it runs super now, got rid of a couple goofy idle characteristics as well. Anyway, I saw 20W-50 and couldn't help but share this un-related story

The reason he gave me is that with the high mileage on the engine, the heavier oil kept the oil pressure back in the same range it ran for the first 100,000 miles, in spite of the greater bearing clearances. He also believed the thicker oil clung to moving parts better and made the next cold start easier.

He was not a professional mechanic. I gathered that his Father taught him this practice back on the farm.

Maybe not, but its what the owners manual says to use under severe conditions, I figure my Jeep is under severe conditions 75% of the time, so its what I run in both. The front you have an option though.

That's the same as I was taught by older racers/mechs in the 50's and 60's. I doubt they had any education or training, just experience with what they had. I ran 20W/50 GTX in all my Chevy hotrods in the 60's and 70's. We thought it was the "thing to do" Ron

Just changed mine after putting some miles on after the gear change.....75/140 + additive in the rear.

80/90 in the front.

Mopar stuff.

Good to go.

No additive needed in the rear on the Rubi, the LS is gear type, not clutch. And, if you used 75/140 sythetic, it is already formulated with friction modifier.