50 weight motor oil works well also. I am always amazed at people who never change the oil in the trans. ATF has no side effects on a mopar 4 speed. Only reason they make more noise is 90wt dampens the sound of worn out rollers, wore out mainshaft tips and worn down countershafts. Really if your going to run a 4 speed call brewers or passion and get a quality complete rebuild kit and replace the parts that are out of spec from 40-50 years of use. It seems that lots of people claim they rebuilt a mopar 4 speed but can't read a micrometer or dial indicator.:banghead: these are the same people rebuilding motors with a bag of dope and a gardenhose. Most mopar 4 speed woes can be solved before they start if you check some basic things but 90% of people don't. Did you check mainshaft to see if it was bent ? (dial indicator). Did you check mainshaft tip for wear ? (dial calipers). did you check countershaft ? (dial clipers). did you check countershaft endplay ? (feeler gauge) ? funny huh! answer is usually "huh ? ummm yeah no let me bring it by for you to look at"
