On sliding surfaces, such as pistons, valves, tappets, springs, gears, etc. I use a molybdenum compound made by Kalgard.
I thought for years that Teflon was the answer but found out that Teflon had a cold flow problem (doesn't stay on the part). It can squeegie out from under pressure and heat.
In the exotic material machineshop that I worked at I machined teflon but it wasn't until I machined Molybdenum that I started reading about it's properties. I never had a problem holding on to Teflon but when finishing Molybdenum parts, they seem to feel oily in my hands, even after degreasing with Trichloretyhelene.
Molybdenum is in the chrome family of metals. Doesn't have a cold flow problem.
On rings, you see chrome rings and moly rings available. Both have about the same coeficient of friction but chrome doesn't pass heat very well. Moly passes heat about the same as castiron but will last longer and have less coeficience of friction. Your piston passes it's combustion heat mainly through the rings to the cylinders. Thus your best choice would be the moly rings. For years I've stocked rings with a chrome top ring (for the lack of lubrication) and a cast iron 2nd ring. This made a longer lasting topend but moly should beat this. Dual chrome rings can't pass enough heat sufficienlty under some applications and can lead to a runaway heat problem til the piston crown collapses'
Kalgard sells/sold 2 types for ferrous and nonferrous applications.
I've used it as a thermal barrier in the exhaust ports, piston crown, combustion chambers.
Reducing friction on piston skirts, cylinder walls, shafts running on bushings, valves, guides, rocker shafts, shifting linkage, anything that slides.
Don't use it on ringlands.
Whew! Another Jaegermeister for me.