Why does my 1979 Mustang burn oil in high rpms?
Well the only 2 possibilities are valve guides or piston rings, and the way to tell is with a compression test. You can repeat the test with a table spoon of oil in each cylinder, and if it improves, then it is the rings and not the guides. Guides are a lot easier to do, because you only have to take the head off.
For rings you really need a whole rebuild.
However, it is possible the rings are just carboned in and sticking, but I don't know a good way to clean them.
All I can suggest is switching to Castrol 20w-50 oil, and frequent changes to clean it out.