My 1-cent worth of my opinion.
I rebuilt two of these, one was using 30w oil and the other had ATF. There were no differences in the amount of wear or evidence of ring slippage. Both transmissions had the same problem – no reverse. (Broken reverse forcing cone) due to driver abuse. So my point is this.
If you ever get the chance to rebuild these transmissions, and see how they are made and how basic these things are, this debate would never happen. Three things are going on in the transmission, 1st is the flow of fluid, 2nd is the pressurization of the fluid to activate servos and pressure plates. The 3rd thing is lubrication.
So what are we looking at, 1st we need a oil that flows well and keeps it viscosity with little foaming, 2nd we need an oil that won’t break down under pressure, 3rd the most concerning the lubricating factors. We want an oil that lubricates well but not too well to where the bands start slipping. So what oil has all these three factors, Lets keep it simple for a simple transmission. NON - DETERGENT MOTOR OIL and ATF.
There has always been a debate of what oils to use in these early GMC hyromatic. Remember the internal combustion engine was only developed some 47 years before these came to production. Oil technology and development was at best primitive to today’s standards. Remember they had no computer labs or elaborate testing modules, 90 % of testing was trial and error. Since the oils/lubricants they had at that time were only developed to level of different weights, i.e. 10w, 20w, 30w, and so on, engineers developed a transmission based around the oils available (standard motor oil). Since non-detergent motor oil (doesn’t foam) was it was a perfect choice to use it in these hydromatics, it had all the three good qualities of what a transmission needs. I always approach my M-211 with a keep it simple attitude. They developed these trucks to be simple out in the field.
So if you are having problems with these transmissions, First do the basic inspection to see if the bands are adjusted right, then flush them to see if any engine coolant is getting in them. 50% of problems with these transmissions results from being out of adjustment, or serviced wrong, so check your rigging, and the other 40% of problems is thinking we have a problem.
These trucks will hic-up, slip, burp, or what ever on occasions. We just don’t have the modern day technology such as the control modular (cars computer) or fuel injection, which keeps adjusting our engines and transmissions every half-second to prevent the hic-ups, burps, and so forth. Put it simply, if it starts and the engine sounds relatively smooth, and you put it in gear and it moves without excessive engine rpms, and you put your foot on the brakes without using excess force and it stops. Then you can say life is good!!!!!!, This is all you can except out of these trucks that are 50 plus years old.
Steve
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