I switched everything over to Amsoil synthetics when I got mine with 150,000 miles on it and have had no issues. You don't have to flush anything, just pull all the plugs when the fluids are hot and the gunk should still be in suspension. DO NOT FLUSH THE TRANS!! For most that do, that's when the problems start. Just pull both pans and drain, replace the filter and gaskets, and drain the trans cooler.