I've tried both the MTL and MT-90.  Here in Las Vegas, NV in the summer (>90
Fahrenheit, up to 115) the MT-90 is superior.  The MTL was too thin and
didn't offer enough resistance for the synchronizers.  However, when the
temperature drops below 50, it's a little thick and the shift into 3rd has
to slow down or you get a clunk.  Down below 30 it's definitely too thick
until very warm.  No perfect answer for all conditions.

Bill
'86 Syncro for 10 years