T5Z grinds...

l, I have a T5Z in my Lincoln Mark VII, it ground going into 4th. I changed the fluid, not thinking I used Mercon V, it ground 5th, 4th gear issue gone...just moved to 5th... Okay, I drained the Mercon V out and put in regular Mercon. It still grinds, I rebuilt the trans a short while back, which doesn't make me feel good about it grinding, I used all new synchros and bearings and such, end play perfect, the whole 9 yards. It shifts great, but 5th, I have to let the gears slow down a few seconds before I shift, which sucks, but what can you do, right?

Down to the question, what fluid is best? Is there a fluid that lets the synchros "bite" a little better? I've hears 10w-30 can be used, and several people have had lots of luck with motor oil, but I don't want to mess something up. I've also heard of the GM synchromesh, but with fiber lined synchros, that's a no, so I won't run that.

Any opinions? Thanks!

But why did the grind move a gear with a fluid change?? 5th gear is on the output shaft. I've have had a Steeda tri-ax on this transmission since day 1. And I don't slam it into 5th gear, maybe quick shift 1-2 or 2-3, but 3-4 and 4-5 are easy shifts, how could it be a bent fork when all other shifts are smooth as silk? #5 was perfect before the change, now it grinds. Seems to me the synchros are too slick and can't let the gear slow down in time to mesh perfectly. Hence, is there a fluid that allows the synchros to slow down quicker?

Thanks! That makes sense, I'm going to try a fluid change again, maybe I still had a little synthetic left in the case. Hopefully that'll work... Thanks for the replies!

Okay, I changed the fluid, I went with castrol Dex/merc. I also read of other people having a 5th gear grind, sometimes caused by shifting into 5th gear at an angle. I think that could've been some of my problem, I shift into 5th by pushing the shifter all the way to the right and then into 5th, so far, no grinds.