I’d begin with pulling the plugs to give a visual inspection, and then perform a cylinder leak down test. If you don’t have those diagnostic tools, just do a cranking compression test to see if anything is weird, if the numbers are low, squirt some oil into the cylinder and see if the psi comes up, if so you have a ring seal problem. I’m thinking the engine has too much lower crank case pressure and blow by, it’s most likely the rings, and then even sloppy valve stem to guide clearance and shot valve stem seals are all part of oil consumption problems. Bottom line the engine isn’t healthy anymore and it’s only going to get worse.
