Quote:
Originally Posted by duke36
Yes; the closest modern gear oil found, based on inspection of an original "600W" can is 250W, such as Penrite's or Lubriplate SPO 299. These are OK for yellow metals without additives. The ZDDP additive is usually added to engine oils to increase zinc levels but could damage catalytic convertors. That's our understanding why STP discontinued the zinc- rich red bottle in the US some years ago.
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Lubriplate SPO 299 is ISO Grade 1000; they also classify it SAE Gear Oil 250 Heavy. That is quite thick. Probably too thick.
Lubriplate SPO 288, with ISO Grade 680, SAE Gear Oil 250, is closer to the original 600W. This may even be too thick for cold climates, until the transmission warms up. But if you know what's happening and adjust shifting appropriately, it's OK.
The Ford Service Bulletins mention diluting the 600W with 10% kerosene in the winter to thin it out a bit. I don't think I'd bother with that, but you can (if you are driving your A in Minnesota in the winter!). Or use the thinner Lubriplate SPO-277 (ISO 460) in winter, only in the transmission.
With an ISO Grade 680 oil in the transmission, the gears in the transmission will slow down their spinning very quickly when you step on the clutch and shift to neutral. It makes driving and shifting a pleasure. You do not need to double-clutch when going from 1st to 2nd and 2nd to 3rd gears. You only need to double-clutch when going from 3rd back down to 2nd gear at speeds above 15 MPH.
MANY Model A transmissions have been filled in the past with "modern" gear oil that is way too thin (like SAE 90 or SAE 140 oil). This was mostly done out of ignorance of what viscosity the original oil was, and the guys used what they had on hand. They did not have an SAE 250 oil (probably didn't even know it existed!). As soon as you do that, the shifting problems begin. You have to double-clutch on every shift because the gears spin too fast for too long, so they clash when you shift. Thus began the old wive's tale that you had to double-clutch a Model A all the time. Not so.
Answering the Original Poster's question, STP is not a stand-alone lubricant, it is a thickener additive. It is not intended (per the manufacturer) to be the only thing you put in a gearbox. They say add 10% STP to your manual transmission oil. Sure, running straight STP may work OK, but why do that when the proper ISO 680 (SAE 250) gear oil is available?
YMMV.