It's also difficult to tell if you actually have any oil remaining in the oil tank on some engines at windmill rpm's. The B737 CFM56-7 has a habit of not scavenging the oil from the engine back to the tank at those sorts of low rpm's, so the oil tank quantity can show all the way to zero, even if the correct oil quantity is available once the engine is lit off and the scavenge pump picks up its game.
Just a thought !
Cheers, FD
