Diesels are different from the gas engines in that the intake side is free air so dilution with more 'free air' makes not a jot of difference, as it might, to the mixture of a gas engine with (carburettor).
The diesel should be running with a minimum vacuum at the intake side (not like a gas when the carb throttle valve is closed completely), limited to the state of the air cleaner, so oil is not sucked down the guides, but it can be blown down with crankcase pressure (another possibility?).
If the valves or guides are worn so much that excessive leakage is occurring, then likely it is getting close to the time for a cylinder head overhaul at the minimum. Not as bad on these slower revving engines as on the higher revving modern auto diesels (but they can go a long time without attention these days). Loose valve guide clearance will eventually lead to worn seats/valves. Then things, as in problems, get compounded.
But some engines are more prone to this and lots are fitted with seals anyway
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