Well after 45 years and over 300k miles of service the "four on the floor" gearbox in my '75 Capri is beginning to show its age. 1st and 2nd sometimes pop out on overrun or at slow speed / high load. Shifting from 3rd to 2nd has a nasty crunch except at low speeds. And when hot it sometimes "packs up" and no gear will engage, it usually takes 3 or 4 attempts for it to finally go into gear. I have changed the fluid several times now with the proper specified mineral 80W GL-4 oil I have managed to buy a good gearbox which has less than 60k miles from a friend, who swapped his for a 5 speed about a year ago... Now here is the question, it has no problems, do i change the fluid and put in the spec'd 80W GL-4, or do i go for something more modern like a synthetic 75W-80 or 75W-85? Also, while the diff also has +300k miles it appears to be in perfectly good health. I have changed the oil twice and has the spec'd EP90 GL-5 in it. It's 4 years now since i changed the diff oil and the gasket is leaking just a little bit of oil. Would you keep using the same EP90, or maybe something like a 75W-110? I understand that years ago the 90 grade was split in half, and that the old school EP90 usually had a Kv @100c of between 20-24cst, whereas modern 90 grade gear oils like 75W-90, 80W-90 and EP90 tend to be in the 16-18cst range.