mixing trans fluid?
I would just go with ALL of one or the other.
I'm in north Jersey and I use Mobil 1. As you've heard; when I used all Synchromesh it shifted better than the Mobil 1 once FULLY warm, but it grinded and was VERY 'notchy' when cold (and especially when colder outside).
Since mine is also an only car/daily driver, and is always kept outside, I elected to go with the M1.
IF I had the coin to throw away, I would change back and forth between M1 (fall/winter) & Synchromesh (spring/summer).
But this is all relative and almost splitting hairs since (in stock/factory form at least) the T56 is NOT known as a 'butter smooth/snicky' type of gearbox anyway, regardless of the fluid used.
But the results I hear about the D4 are just too mixed to give me that confidence, so I stick with the 'known to be safe' Mobil 1.






