My truck has an early 4.14 first gear ZF 5 speed with God only knows how many miles on it. It was put in used when I got the truck.

The transmission is very loud at lower rpms and I'm sure it being so loose is what makes it so loud.

My question is, can I put a heavier viscosity oil in it to maybe quiet it down some and if I can what weight oil should I try?

It shifts great and is quiet when the tach is above 1500-1600 rpm. Thinking about trying a 5-20 synthetic. But I am waiting to hear from the experts on here before I do anything,

I don't want to go through something like I did on the Autolite GP deal.

 

Don't put anything other than ATF in it. A good full synthetic ATF is probably the best you could do for it. Do you have a solid flywheel or the original dual mass flywheel? If you have a solid flywheel, then the noise you hear is gear rollover, and there is really nothing you can do about it. If you have the original dual mass, I suggest getting it out of there ASAP!

 

Also over filling the tranny by a qt. or 2 threw the shifting tower helps quiet the gear roll over.

 

I have a solid Southbend flywheel. It had a brand new dual mass in it when I got it. About three or four weeks after I got it I went to Swamps and got the stage ones and 6 position chip. On the way home I got up to about 75 mph on the interstate, turned it up on the number 6 position and went wfo, when the boost got to about 25 to 30 lbs the tach went straight to red line and I about didn't get home. I had to get down to first gear to get in the driveway at the farm. Smoked the clutch..........

I will add a couple of quarts of ATF and see what happens.

Thanks