So, as a corollary to the gearbox oil discussion, suppose a Triumph owner - let's call him "X" - decided to switch from 30 weight non-detergent oil to Redline MT-90 in his 1975 TR6 with J-type overdrive. How much of the old oil needs to be flushed out, and how would Mr. X go about making the swap painlessly? Jeff X Corrosion Acres, Ga.