[Mister Moose]

I would define lugging as too high an intake manifold pressure for the current RPM. Combustion impulse forces get transmitted down to bearings much more when lugging. It's not just oil film, it's the beating. Just like detonation can ruin an engine even though lubrication is adequate.

It's going to be a function of shift speed, yes, but also load. How steep is the hill you're on? But it's hard to quantify with x rpm at x grade at x speed with x weight in the car.

However, if you step further on the gas and you get an immediate increase in speed, you are not lugging. If you step further on the gas and get no increase in speed, or the increase is painfully slow, you just might be lugging.