valve stem seals, oil drain back holess

Those are the passages in the cylinder head that allow oil to run back down after lubricating things up on top. They only get plugged if the engine is full of sludge from waiting way too long between oil changes, if the engine is running too cold from someone removing the thermostat, or there's excessive blowby past the piston rings. When sludge prevents the oil from draining back down fast enough, that oil can collect on top and overload the valve guide seals resulting in blue smoke from the tail pipe. That's more likely to happen at highway speeds. At lower speeds the oil has time to run down.

A trick you might try if you suspect heavy sludge buildup, is to add a quarter a quart of automatic transmission fluid to the oil, run the engine for a day, then have the oil changed. The transmission fluid will dissolve a lot of the buildup better than the detergents in the oil which might be worn out.