You really don't have the option. If your tag or axle cover states it uses synthetic, you must use synthetic. Most if not all dealers stock the motorcraft friction modifier. One of our wrenches has a 03 MM and is going to see DAVE in the next couple of weeks. He has 4.10's coming and I can guarantee he will add synthetic and friction modifier.
Yes, if you go to long without it, you clutch packs will stick together and become one solid POS...
Sorry, ment to post as reply not new thread