This is for the six speed ( manual ) trans folks .... I am always doing things to be good to my C5 .... it turns out a good my have been a bad .... sometime ago when i replaced my clutch i decided i would also replace my trans oil .... so doing something good ? i used a syn. oil .... about a month later i noticed the trans was getting hard to shift .... since my C5 has a lot of miles i thought syncros getting worn or the clutch had bad parts .... well truth is the syn. trans oil turns to loose goo when exposed to high speeds and heat over time .... thus not letting the synros do there thing .... thus hard shifting .... changed to GM gen3 ... shifting is better now .... just thought i would share my find ....