When should I suspect that valves are not sealing

Question

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?

John P Hutton · Answer

Knowing that a Ford engine has gone 1.1 million miles with proper care; why subscribe to letting you think something other than performing maintenance might lead to a known point where once damage has occurred: one might get the vehicle back into a fully functional operation short of correcting all the damage to the point of all specifications restored.  Obviously I don't concur with negligent auto care.
But to your point, that kind of oil consumption means the rings are worn out, the seals then had oil being pushed out them from excessive blow by from the rings. And following that the valves got oily and started caking carbon, losing compression further.   If you want to conserve an engine; never overheat it.

When should I suspect that valves are not sealing

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