RTV vs gasket
I recommend using RTV, aka silicone sealant, in place of a gasket whenever possible. I worked at the Ford prototype engine build and dynamometer facility from 1978 to 1985. During that time we worked with all the major gasket and sealant suppliers looking for better, mostly cheaper, ways to keep fluids in and dirt out. RTV was the easy winner for diff covers, oil pans, valve covers, timing chain covers, etc.
Most important is to make sure both sealing surfaces are chemically clean. Use brake cleaner, acetone, etc., anything that does not leave a film when it dries will work. If resealing an old RTV joint, you must make sure all the old RTV is removed. New TRV simply will not adhere to old RTV.
The best proof I know is a 650 Triumph I built using RTV in place of every gasket except the copper head gasket. The only place it leaked oil was from the trans output shaft and that was after about 60,000 miles in three years. Considering how Limey machines usually leak, I acknowledged the leak as an automatic chain oiler and let it go at that.
Mike in Michigan