Ok so let me first explain WHY i put 0w-20 in my clapped out 150k engine.
I wanted to kinda clean it out a bit without using any solvents or kerosene
So i thought i would just try running a thinner oil for a while.
Unfortunately the only 0w-20 near me was full synthetic.
The engine is in pretty good shape it has a newish radiator and water pump and new oil sending unit and it only has a slight main seal seepage, normally.
I normally run Castrol 5-30 for high mileage vehicles and it works well the main seal seepage is slow enough that it doesn't drip at all.

Well two weeks ago i changed the oil and filter and went with the 0w-20 for cleaning.
Here's what happened.....
THE GOOD:
Within a couple days the oil turned dark. "That's good" , i thought "Maybe the thinner oil is cleaning things up"
Surprisingly the engine actually started running quieter not louder as i had expected.
The idle seemed to smooth out as well. Normally while sitting in the vehicle with it idling i can feel a random vibration when idling well the 0w-20 seemed to quiet that down quite a bit.
But here's the kicker.. I went from an average of 18mpg to 21mpg!
Maybe the thinner oil offered less internal resistance to the moving parts maybe the lash adjusters were able to move a little faster...maybe i gained a tad more horsepower.. Who knows.

THE BAD:
After running the oil about a week i noticed when i would restart the engine after sitting a few minutes i would get a singular PUFF of blue smoke .. Apparently some of the thinner oil was making its way past one of the valve seals. Possibly the worn seal was affected badly by the full synthetic... Who knows.
After two weeks I decided to change the oil and filter and put my regular oil back in so i crawled under the truck to change the oil and noticed that the oil pan and transmission were very much more soaked in oil and there was a drip forming. Again maybe it was the thinner oil maybe it was the full synthetic or a combination of both.

THE WEIRD:
After running the 0w-20 a couple days i started to get a P0128
Apparently the engine was now taking too long to warm up.
After the radiator and water pump change out i did a few months ago the truck does run cool but after switching to 0w-20 i noticed the the temp gauge barely gets off the the second mark when warmed up.
So i guess the thinner oil is causing less mechanical and fluid resistance in the engine keeping it cool... Again.. Who knows

So..
Would i suggest any to run 0w-20 in an older engine for cleaning purposes? .. A: Sure
Would i advise running it all the time? ... A: Absolutely NOT, not unless it was a bran new engine.

Anyway, that's my recent experience running 0w-20 in a 150k 3.7
Everyone have a good weekend.

 

0W-20 is actually dealer fill for the 4.7L. I've been running it for about 6 years. Currently at 340K mi. It flows to the hydraulic lifters quicker. I don't burn any oil between changes, which is every 7.5K. I replaced valve stem seals in 2020. If you've got a bunch of leaks or a worn engine, yeah use 5W30.

https://dodgeforum.com/forum/2nd-gen...-the-4-7l.html