I thought anything thicker than 5-30 was bad. I read a while back that the tolerances in the getrag are so close that only thinner oils could lubricate it properly.

I would only use thicker oils in a "have to" situation, though Syntorque isn't AS thick as 80-w90 and it does have the correct GL rating.
It's not "tolerances" that are close in a Getrag, they are a rattly beast, even newly rebuilt, it's the oil passages that are smaller.
The biggest 2 killers is the lube slots on the input, which lubricate the bearing for the mainshaft and the lube holes for the cluster (called "counter shaft") bearings that support the cluster through pin.

Combine those smaller passages with the invariable over-use of silicone sealers, along with overloading and/or underfilling and a formula for "Filings-'O'-Getrag" can result in short order.

Mark.

On the Contrary...
 

Heavy gear oil is about the worst thing you could put in a Getrag, and the biggest culprit in their bearing failures. It's too heavy to effectively reach the bearings and will make cold shifting way more difficult than it needs to be. GL4 is the preferred rating for an MTL. GL5 is the the diff/hypoid gear stuff:

http://stuff.is-a-geek.net/OnlineDocs/Vehicles/MTLs

Add an extra quart above the fill plug

Yea i had 8090 in mine for a very short while not good.. I went to synthetic 30w and it shifts fine and has had 0 problems. I do overfill by 1 quart by taking out top PTP cover bolt and rigging pump to fit in hole. Or you can take the shifter out and just add 5 quarts.

I use 10-30 synthetic oil and a quart of Lucas oil stabilizer with mine.

I used 80w90 in mine for over 10 years until I found out different. I tore it down for for a perceived bearing noise. new rollers and needles later, it was a rear end. Point is, 10 plus years with 80w90 and now bad bearings. Right now I'm running 20w50 with some stp in it. I'm also now running a quart over, as I also only filled to the plug. Damn that was a great tranny. I love my 1st gen.

OK how about for an Auto trans in a 91.5 dodge W350? The one in my picture by my name. It has an overdrive by the way if that helps any but it does slip a lil under heavy loads I think it needs a new torque converter cuz it takes awhile to get up to speed. Any idea's? Thanks

i saw that you have a recent post on youur tansmission is dying so we obviously shouldnt try the 5-30 weight hahaha just saying

For what it's worth: I have a '92 Dodge Ram D350 in my shop with a 5.9L Cummins backed up with Getrag G360 5 spd. M/T. It came in with what sounded like bearing noise in the transmission. After all the inspecting I could do with it in the truck, I drained the oil to see what it looked like. It was fairly dirty and looked a little thin, and it was only about half full to the oil level plug. I refilled it with everyday 85W140. When I drove it the noise was gone. After doing some research I see the manufacturer calls for 5W30. In a truck gearbox?! Please! No wonder it made noise.