Only problem I know of is synthethic fluids cause older types of RTV sealants to disolve. When g.m. came out w/ Dexron VI they issued a service bulletin stating that it would service back to 1949 in all g.m. automatic transmissions but not to use it in manual transmissions and transfer cases that called for Dexron III and earlier fluids. same issue on g.m. rear axles, i.e. RTV cover sealant on 9.5" semi-floating & axle shaft to hubs on some 10.5" full floaters. Have seen some engine that use RTV at the ends of the intake manifold develop leaks after switching to synthetic oils.
