Good compression and on the road high temps equal wear, piston to bore. And some taper. The heat is getting past the rings and heating up the piston. That makes it tight in the bore and adds lots of friction and more heat. If the bore is good then the piston grooves are worn and that will transfer more heat to the piston skirt. You will not see much blow by at freeway speeds. The draft tube is working good at that speed. How much is the oil down at the change interval? If you don't measure both the filter canister and the dip stick you won't know. Worn oil pump will do the same as it gets hotter as well. But to your credit, you have done your homework and kept good records. You might have some worn rings with very little taper or ridge. If those rings got hot they will anneal and lose the pressure to seal. And I know you have done a compression test. Now do it when it is hot and the oil pressure is down. Might be helpful. Water cooled engines are much more forgiving of this that air cooled. If this were my Harley it would be stuck by now. Just some things to think about. You have done the right stuff. Cooling system sounds good. Timing sounds good and if you don't have any stumble then air leaks are off the table. You can have a head gasket issue but that would push the coolant out so that is good to. What is left is posted above.
