It's about as illegit as it gets. There have been numerous discussions by people who think you shouldn't change your tranny oil for very specious reasons. If you follow their logic, you shouldn't change your engine oil, either. The tranny fluid will burn and oxidize over time, until your transmission starts to shift in a jerky fashion. There are NO inline filters in the transmission line, and if you want your transmission to last, dump the old fluid asap and fill your transmission with Mobil 1 ATF. Not cheap, but the best. You will notice an immediate difference in how smooth the transmission shifts. If you're driving a turbo you should also be using Mobil 1 in your engine because of the turbo. Volvo recommends a full synthetic for its turbo models. If you've driven 160K without changing your tranny oil, you should flush at least twelve quarts/liters through it.

Flushing your transmission is simplicity in itself. It does not involve unscrewing the drain plug at the bottom of the transmission. This will only drain out about 2-1/2 liters. The transmission holds around 7, and to do that you have to disconnect the transmission coolant hose at the radiator. If you want to do it, leave a post here and I will tell you. If you think leaving all that burned oil in your transmission is legit, then don't. Dick