Yep RP gear oil is a synthetic. Personally, I would use Red Line in a rebuilt LSD (I did actually on my TRD per above). I went with straight 75W-90, but you can put in a quart of 75W-90 and top off with 75W-90NS. Most of the slipping issues you see with the synthetics is due to guy putting in a high mileage LSD...rebuilt is a different story.