I have a limited slip differential from 1969. I last changed the gear oil in 1986 or 1987. This car has not been driven much in the past 20 years. I am installing a new stroker motor and want everything to be fresh to reduce risk of damage to the drivetrain. Transmission fluid and filter change seems like a good idea. I was also planning to change the gear oil until I discovered there are issues with "new, improved" gear oils in our old cone type posi units. I have some fairly old NAPA Premium Performance gear oil, still sealed in the bottles that I purchased about 10 years ago. The label states that it is for use in "both regular and limited slip differentials." It does not state if it already has an additive for LSD, or even if it is conventional or synthetic. It is rated "GL-5, GL-4, GL-3." The part number is 75-210. I do plan to add the AC Delco limited slip additive whether I use this oil or not.