My second gear will intermittently grind during normal driving (i.e. shifting at 3000-3500 rpm). I just switched to Redline as well. I think I'm going to switch back to regular old dino tranny oil. Anyone know what kind Mazda uses at the factory?
I talked to the Redline tech yesterday, and he said MT-90 would be his choice (I used 75W90 NS). He said the only difference is the additives. I get some gear whine now too, which makes me think the synthetic is too thin to protect the gears, and hence is not getting the synchros up to speed. I never had a problem with second gear before, and I would only occasionally grind third - I was trying to shift too fast.
Oh, BTW...75W90 NS is a GL-5 oil, while MT-90 is a GL-4.