I have no idea where the number "303" comes from, but I believe it started with the John Deere J-14 oil that needed to combine oil for hydraulics and gears from one common sump. 303 oil typically has the same gear protection as GL4 gear oil and tends to have less then 1% ZDDP anti-wear additives that protect gears and bearings. In fact, many blends of 303 oil have less then 1/4 % ZDDP. Modern combo trans and hydraulic oil like Deere Hygard has 3 % to 5 % ZDDP. HUGE difference.
That said, I have come across many off-brands of so-called Hygard equivalents with less then 1 % ZDDP which is kind of a bad joke.
Many old tractors used pure mineral oil with no zinc additives - so when compared to that - 303 is an improvement. But modern oils like Deere Hygard offer a huge improvement over 303 oil.
My 1960 John Deere has a power-shift wet-clutch transmission and in 1960, Deere called for automatic transmission oil. I guess at that time Deere had not come up with its own specs yet.
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