I ran a car from new up to 110,000 miles and did what I suggested to the group. A automotive design engineer suggested the Mobil 1 in the g/box and Slick 50. He said the Slick 50 g/box treatment is worth it as in puts a slippery film around the metal parts. It mixes fine with the Mobil 1 oil with no adverse affects. Do a Google and you will find that people who put this stuff into g/boxes with wet clutches have had clutch slip. The engine stuff he was not keen on at all.
I sold the car to a relative and suggested carrying on from where I left off. He has and is now up to around 160,000 miles. The car still sounds newish, except some belts squeak at bit. His driving is predominantly stop start around town, short runs.