Has anyone here used "Slick 50" or something like that in a thumper yet, and if so how did it work?

 

Save your money. I personally know an expert witness in the class action lawsuit against Slick 50 (and others) Quaker State (Slick 50) basically admitted that their product did nothing it claimed.

 

Here is an older article with lots of information on oil additives, including Slick 50. It has a lot of discussion about PTFE (Teflon) additives.
Snake Oil!

And here is a newer article that discusses additives such as Prolong that use chlorinated paraffins.
http://www.pecuniary.com/newsletters/chlorine.html

Bottom line - stay away from the additives. Modern oils, dino-juice or synthetic, are formulated with a proper mix of additives that are designed to work together.