Well, I drained the ATF and Installed three quarts of conventional 30W engine oil. I just did a road test and it is much quieter than before. All 18 splines are a little louder than the 23 splines. It's something that has to be lived with. With that, I am a big fan of STP. I religiously use it in my engines. I don't see any reason to use it in a manual transmission though? Thanks for all the replies and suggestions. Really appreciate it, especially that chart.