Re: Redline MTL-only if you want the best.
I bought my 82 zxt with 172K miles on it and the T5 would want to grind a bit when it was cold when shifting to 2nd. This would disappear after a mile or so of warmup.
I drained the ATF and replaced it with Red Line ( differential as well) and the grinding went away. I have to believe the tranny is a little more happy with it.
The Manual says either ATF or 90W is ok, although Ford apparently only recommends ATF. Perhaps high mileage trannies will benefit from the 90W.
enjoy the ride