Maybe valve guide seals (different from valve clearances).
I presume you've decelerated to 60/70k buy just closing the throttle, before putting your foot-up-it again.
This engine vacuum could be enough to pull the oil past the valve guides.
To give that theory a real test, find a decent hill to engine brake down, and check out the back when you re-apply the throttle at the bottom.
Oil smoke is generally a bluish-grey colour, over fueling will be blackish with water being white (steam).
Plain old grey could be just 'sh*t' getting blasted out of the exhaust, could pay to check that a muffler or cat hasn't disintegrated, but this would normally have some loss of performance (blockage) and/or some increase in noise.