Do what you feel is right for YOUR truck. As for me, 15W40 in engine, P/S fluid in P/S, 10wt in trans and PTO hydro, 80w90 in diffs and T-case. This should never cause any problems due to fluids. I found that 10wt is a lot less money than Dex III. Besides, if it was good enough for the military and has proven itself why change and possibly open a new bucket of worms. JMHO