I just did a transmission fluid drain/refill on my 06' Xterra. I drained nearly 5 quarts of fluid from an 11 quart system. I re-filled using Mobil 1 ATF (which is Matic J compliant) as opposed to buying Nissan's fluid. The fluid level is where it should be. I have noticed the shifts are a little harder and when I was at Lowe's the other night, sitting still in the parking lot going through the gears, I heard a distinct "clunking" noise that I never heard before. I've read some people on the web say use only Nissan Matic J instead of any other fluid that just "meets Matic J standards". How much better can Nissan's Matic J fluid be compared to Mobil 1? Are the noises I heard related to using a different fluid?