"The Swiss grease their bolt rollers. You can look at SwissRifles.com, Guisan** provided lots of excellent advice on the Swiss roller bolts. The Swiss cleaning kit had two grease containers. I purchased several of these and I believe the black grease in these containers is a molybdenum disulfide grease. The bolt rollers are heavily coated with this grease. Considering the high contact pressure between the rollers, the bolt head, and the trunnion, I am of the opinion that grease or LSA is the appropriate lubricant for these rifles."
"Automattenfett" ('grease for automatic') is the key word. Sadly, we can't seem to import it for some dumb reason; it's considered an article of war or some such
. It is a moly-sulfide, but the exact mixture & optimization isn't really known, and since wrong-lube issues have proven disastrous in STGW57/PE57 rifles before (the recesses cake up and the rollers lose engagement if oiled), any experimentation should be done with a keen eye.
A big chunk of why the G3/etc are so reliable against debris is that hellacious spring that pops the rollers out when in battery. I theorize it was added late in the game during design when the shift to 308 (vs 280 Brit or something similar) was made, to 'cheat' the action the last little bit needed for safe function without having to radically redefine the geometry of the action. But the effect is a bolt that rams itself into battery way harder than any recoil spring or bolt momentum could accomplish, which means that a stoppage slightly out of battery is basically impossible (I suspect it is strong enough to size out of spec brass, or embed debris into case walls as it cycles closed). Sadly, it also makes some aspects of service obnoxious to impossible, but luckily no one thinks they ever need to take their bolt apart for maintenance, so...
"That pic doesn't look like a normal G3 or HK91. The hand guard hanger should be welded with a fitting to the cocking tube, not (apparently) just stuffed between the barrel and cocking tube."
Not an expert on HK's goofy cocking tube design, here (the Swiss design was way simpler & moots the entire need for the thing*) but this was the only good picture of the area I could find. Probably a home-build or something if it is a non-standard configuration, so perhaps a upload of someone's HK triple tree area is in order?
"My gun chokes on tar sealed ammo."
Yeah, the Swiss used wax on their 7.5 during the STGW years for this reason. There's just no way around those precious flutes delivering various bore fouling back into the chamber area.
TCB
*instead of spring loaded rollers, the locking wedge simply pushes the rollers out when the bolt reaches its forward stopping point at the chamber. This means there is no camming needed to cock the gun, just pull back on the locking wedge with a handle. The anti-bounce scheme (part of the function of the HK garage-door spring) is also very clever; the brass is made with a single neck, but the chamber has an extra step in its neck. The corner of the brass's shoulder actually gets swaged down about .05" when chambered, cushioning the action against bounce (and greatly contributing to full auto control, I'm told). Makes the brass weird for reloading --you'd want to fire form it back out to the original state in a bolt action next-- but being as it's Berdan it is no huge loss.
**RIP; I'm pretty sure no one on Earth knew more about Swiss guns than that man