I have used STP for assembly lube for decades. Best thing in the world if you aren't doing a rebuild and reinstall to run in 48 hrs. Everything else will run off in no time, you can take the engine apart weeks later (without running it) and there is still a coating of STP on everything.
With newer engines running tight tolerances and new oil technology, I've stopped running additives at all.
When I had a fleet of rental VW Vanagon campers (water cooled)in the early 2000's, I always used used STP to rebuild, but with the first oil change I would add a teflon additive like Tplus or T-fal I think were the names.
That got a lot of customer abused buses home, badly overheated, low on oil, pushed to the limits, that normally should have stopped running.
The engines I bought for rebuilding had been through the same thing but they had spun bearings, seized pistons etc.
Just my experience.
