I am going to change the transmission and rear differential fluid on my Ranger. Ford specs the manual transmission for 2.8 quarts Mercon ATF and rear differential (non limited slip) for 5.3 pints 80w-90 gear oil. I am hoping to get some input as to what oil to use. For the manual transmission, would I really feel a difference using Amsoil or Redline product that seems to be recommended so much? It shifts pretty smooth now, but will using a high quality synthetic make it shift like butter? For the rear differential, would using a 75w-90 gear oil do any harm? That weight seems to be more popular and readily available. No heavy towing/hauling over 600lbs will be done. This is my daily driver and I just want the fluids to be fresh and doing their jobs. Will I notice a difference using the cheap stuff vs. the more expensive synthetics? I plan on running each for a minimum of 50K miles. I am pretty easy on cars, so it is not like it will be abused. What do you suggest?