I've never heard of mixing it.
I think you should go with straight synchromesh.
I works great when it's really cold out.... No grinding of gears.
Just an FYI...
Loosen your filler plug before removing your drain plug.
If your fill plug is seized you won't be able to get any oil in if it's been drained.
I used Redline MT90, I couldn't find synchromesh.