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You are missing a few points of obvious logic. 1, A gl-4 oil may meet lubrication minimums of that level, by using a lubricant addative ofther than sulfur.. which is one o fthe ones that may harm yellow metals. All it would take is a reformulation and different lubricant. For instance.. the pennzoil PDF file specifically adresses the fact that their gl-4 oil was designed for applications that wanted/needed the extra lube of the gl-4 spec.. but needed yellow metal protection , which is not commonly found in other gl-4/5 oils. It's in black and white.. take a look. The fact that the 9n and 2n manual stated one oil, and the 8n manual stated another oil is most likely due to the fact that they spec'ed whatever was the common oil on the market at the time. Remember that there was a bit of time lapse and oil advancement from 1939 to 1947. Another curious point is the rear end parts are identical for the N's... same ring and pinion.. therefore oil / requirements should be the same.. If you took two identical parts.. and placed one in a 9n, and one in an 8n.. there is no magical reason why one would be limited to gl-1 and the other could magically use a ( gl-3?) mild ep oil, just based on the name of the machine they set in. As for gl-3.. Llamas seems to hint that the tractor supply oil for ford tractors may be mildly ep.. due to the sniff test.. I'd guess this might be a gl-3.. absent any other specs on it... hard to say. As for ep oil like a gl-5 inthe system. I know lots of people do it. I do believe there may be marginal yellow metal damage occouring. Whether this is enough to show up in a few decades.. hard to say.. I'm not a metalurgist.. or petroleum engineer. It would be interesting to have an oil anylisis of a N using gl-1 for a year, and then gl-5 after a complete sump cleaning... then see what the metal numbers show.. Soundguy
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