my 2002 mustang gt grinds into third gear only at high rpm and when shifting fast. i can shift fast at normal speeds and it will not grind. the only way to shift into third at high rpm without it grinding is to shift a bit slower. Is it the synchros or can it be something else.I also have a short throw so would replacing the shifter change anything.
It is the synchro ring. If you nip it in the bud you can get away with just new rings. After it has been scraping a while (metal to metal) the synchro hub itself will need to be replaced and even possibly the gear as well.
Typically, third is the first spot to show the wear, but any can go.
I strongly recommend sticking to one of these three: Mercon-V, Mobil-1 Synthetic, GM Synchromesh (or Pennzoil equivalent).
The official fluid is the Mercon-V. I understand Tremec developed and tested the TR3650 using the Mobil-1, and Tremec support has recently been recommending Synchromesh. (Not to be confused with RP Synchromax which is not the same)
For an 01-04 TR3650 the fluid should be 3/4 inch below the fill plug per a Ford TSB. This is to reduce drag caused by the excess fluid on the gears. Primarily this addresses a tendency to scrape when the transmission is cold.
The fill plugs were lowered in the redesigned 2005 TR3650 so this does not apply to the newer models.