I just lost an (old, 230K miles) engine with no compression in two cylinders. It had been drinking oil liberally for a while, but just prior to this failure, had begun seriously drinking oil (6QT per tank of gas). They said that a likely failure was burnt valves, but since there was no compression, there was no reason to investigate further than the compression test.
I had not done well in changing oil for a while, essentially going downhill after it had been leaking/burning/whatever more than 6 qts between oil changes. Arbitrary, unwise, cringe-worthy, to let an engine go without oil changes for too long but there it is.
SO, my question is this; assuming it was burnt valves (50/50 shot, given that the engine had been seriously drinking oil), at what point of oil burning does it become a lost cause and likely to harm the engine seriously just because of the oil burning? And at what point should I have tried to figure out if my valve seals were OK to prevent burning my valves and prolong the gradual slide of performance and oil usage increases?