Didn't even know there was a "high mileage" variant for GTX. Just checked my inventory and am using the standard type, which is around $4/qt. from Walmart. I would imagine any fossil engine oil would work as well. I used $1/qt. (at the time) generic 10w40 oil in the primary of my 1996 RK and never had any primary trouble past 100k miles, that is except a seal that was replaced at 65k. Of course that was an Evo, which is an apples/oranges comparison, and I did change the chain, stator, and clutch disks on that bike with the seal replacement as a precaution even though none of that needed replacing at the time. I used an Hayden M6 chain tensioner for a while until the springs broke at around 85k, then changed to the HD self-adjusting type like what is standard today when it was introduced. Never had any trouble with that tensioner design on that bike or my '07.

IMO the primary is a low-tech environment that doesn't require a high-tech oil, and in fact something with lower levels of friction modifiers should be advantageous considering the needs of the clutch disks. M1 VT 20w50 is probably okay since it has lower levels of some friction modifiers like moly in its formula, but I think even that is overkill for the primary. I definitely wouldn't use most of the high-grade synthetics like M1 15w50 and especially Royal Purple or Redline in the primary with their high levels of moly. Moly is a great lubricant and IMO a big advantage for engine use, but it makes the oil too slick for the clutch disks.

Having three holes for drivetrain lubrication is a PITA to service, but IMO is great because we can choose the right lube for the job. The engine, tranny, and primary have different needs and thus ideally require different lubrication formulas.

 

HM just has more detergents.

 

Yeah, I just looked it up. That stuff comes in a green bottle while the stuff I've been using is white.