I have replaced the rear differential fluid in my XK about 10 times now and have always used RTV. I clean the differential cover and the differential housing. I apply an even bead all the way around the differential housing and put the cover right on. I torque the bolts by hand, I dont need a torque wrench for those bolts to know how tight they need to be. Then within an hour I fill it with new gear oil. I have never once had a leak nor have I had any issues using rtv. If done right rtv is better then a regular gasket.