JackAttack,
I know this has been a controversial subject in previous posts concerning gear lubricant for the 3 speed and 4 speed transmissions in these AD trucks. It has been my experience, especially with the old cast iron Muncie 3 speed transmissions that have cone type synchronizers that the GL-5 80W-90 gear oil is too slick causing the synchonizers to slip thus causing double clutching for gear engagement. These sychonizers in turn wear much faster. A few years ago I spoke with a GM line worker that worked for GM Atlanta, GA. plant in the late 50's. He was the person that convinced me that the straight 90 weight mineral oil based gear lubricant was the way to go. He told me that GM used this mineral based gear lubricant up until the mid-seventies, even in the Muncie M-21 and M-22 4 speed transmissions. He explained that the mineral oil based gear lubricant had a higher level of ash which in turn caused these synchonizers to slow and engage as designed, thus eliminating double clutching. I purchased my straight 90 weight gear lubricant at NAPA. You have to ask the guy behind the counter and they usually have it in the back in "dust" covered gallon jugs. This gear lubricant is also used for antique tractors. I had a this problem with an old Muncie 3 speed in my '58 Chevrolet Biscayne. Problem solved after changing to straight 90 weight mineral based gear lubricant. Just letting you know my experience. Take it for what it's worth, it your choice.
Tim
