You are misinterpreting what the book is saying about the 50wt. The book says to "top it off" with 50wt. What comes in that transmission is 00 grease. It is a grease that has the consistency of snot. It is somewhere between grease and oil. It uses this material because conventional oil will leak. So what the book is saying is that if it is low, use 50wt to fill it. It won't get low unless it leaks out which is unlikely with the grease. There are two ways to change the old grease out which should be done once in a while. One way is to remove the inner transmission cover and scrap out the old grease. This is the best way. Then add new grease. The grease mentioned above by DeeJay is synthetic and will not tend to glob up like the OEM grease. Another method is to add some ATF (Automatic transmission fluid) to the transmission and then drive it for 40-50 miles to get the box good and hot. Then drain it. The ATF helps loosen the old grease. I would let it drain all night long. In newer Bullets (2003 and later) they changed one of the seals which helps them retain oil, so youmay get away with regualr 90 wt gear oil