I always use Oil.
Thats what the owners manual says use.
That is what the engineers said use.
So... that is what I use.
In the past 50 years, I have tried all kinds of additives...none of them ever did much good that I could tell.
Years ago, I used to occasionally use Marvel Mystery Oil for a few days ... just before I was going to change the oil.
I had been told it was a real high detergent supplement, so I figured it would help keep the internals of the engine clean.
I don't know if it ever really helped anything, but I don't think it ever "HURT" anything either.
When I was putting together a standard transmission or putting gears in a rear end.... I would add just a few spoonfuls of STP.
That stuff was so slick and hard to get off of your hands... I figured it would be good to stick to the gear teeth.
I would put in in a rear end
"ONLY" if it was
"NOT" a P-Trac or a locker,
I figured it would screw up the clutches on a P-Trac.
Just my 2 cents worth.