If it has a fill over-flow hole and the car is level, then you fill it until it comes out the fill hole, that's what it's there for.Just changed my transmission oil and diff oil. The diff oil stunk real bad and there was a build up of grey paste. The transmission oil looked dark golden and didn't have a smell, however when refilling I almost put a gallon in. Just over 3/4 of a gallon went into the tranny and it still wasn't overflowing like I have seen people say. It wasn't leaking anywhere either. So I stopped short of throwing the whole gallon into the tranny and I guess it's ok. Anyone got any advice?