some may have been following along here but i am having a rough time diagnosing a trans issue. i have an 09 mn6 ls3. 12k miles (dont laugh) i wished for improved shift quality (again please dont laugh) so decided to try royal purple synchromax. couldt get 3rd gear under heavy throttle, and 5th gear had a grinding upon entry from 4th.

so i changed it out to Amsoil Torque drive at the recommendation from someone in the forum. no one seems to have these issues with these fluids upon much research in the forum. some describe the shifting to be "notchy" but im not sure what that means. to me if feels like its grinding. the amsoil shifts better under heavy throttle in all situations but i am still get that grinding sensation upon upshifting into 5th from 4th gear. if i go from 4th to 6th there is no problem. if i go from 6th back to 5th there is no problem. going INTO 5th FROM 4th is the issue.

i drove with royal purple in the car for about 200 miles. royal purple corporate told me via email that i had to let the fluid work its way in and the problem may resolve itself. pretty much BS. when i changed the fluid from royal purple to the amsoil i checked the old fluid and it had a metallic look to it but no large particles and nothing dreadfully wrong. i never had this issue with the stock fluid. so my question is: is it coincidence that this issue arose when i switched to synthetic or does it sound like 5th gear is damaged?

 

May be an alignment issue with the shifter box, instead of a fluid problem. After a few years of use the rubber mount bushings which attach between the box and the torque tube mount dry out and crack from heat and use and can let the box move around, causing shift problems. I would check those and replace if necessary. Several vendors carry them. There are also rubber bushings inside the box that the shift rod slides through. Those should also be checked for the same reason, they can crack and shred. Those bushings aren't available for replacement and the only fix for them is to buy a whole new box which goes for about $160 through vendors such as GM Parts House.

 

that shift box was loose with very minimal use early on not long after i purchased the car. i went in and changed to a mgw shifter and torqued everything down the correct way. would this issue fall under the GM 100k trans warranty?

 

I don't know about the warranty, but if you put in the MGW without putting in a new box, that could be your problem. I did exactly that when I installed my MGW and it didn't shift as well as expected. I went back in and found shredded shift rod bushings as well as worn mount bushings. I bought a new box and reinstalled the MGW with the new box and problem solved. My car was five years old at the time. I spoke to George at MGW at the time and he said he recommended putting a new box in if the car was more than 4 years old or had more than 25K miles on it because of heat exposure to the rubber bushings. Since then, he now manufactures his own box, and if I was doing the job again I would put in both his shifter and his box.

 

ok, did you have a grinding in only 5th gear? and what are the chances that this occurred after i changed fluids?

 

No, I didn't have just grinding going in to 5th. My shifts in all gears as I remember were stiff and felt like they were kind of forced. Obviously shifter alignment was not precise with the worn bushings allowing box movement. I can't see that your fluid changing could cause your problem unless you didn't get a full fill. The car needs to be level during the fill and if it is a Z51 with a trans cooler, those lines needed to be full. After filling, it's a good idea to start the car and let it run for a few minutes and then shut it down and check the level to see if it needs topping off. After my last change, it took another third of a quart after it had run.