I'm running a 1:1 mixture of Pennzoil Synchromesh (same as the GM stuff) and Honda MTF on my Si tranny. To be honest, I ran it b/c I wanted to experiment...Synchromesh is a GL4 oil and I was concerned that there wouldn't be enough lubrication. However, I was more concerned about my synchros, so I gave it a try. I haven't had a problem with it yet, but there's nothing to say that it won't grind in the future.

I suppose you can always switch back to MTF if it continues to grind...tranny fluid change doesn't take long anyhow.