GRAVELY recommended SAE 30 for Commercial 10A & 12 tractors back in the late 60's. GRAVELY recommended SAE 10W-40 for 5000-Series tractors in the late 70's. Basically the same tranny was used in the 400-Series riding tractors & GRAVELY variously recommended SAE 30, 10W-30, or 90-EP for them.
Because of the clutch design in the Gravely transmissions, most people avoid using synthetic oils in their tractors. Since the L models share oil between their engines and trannies, that type of oil often isn't used at all. That said, I know of people using synthetic oils in the Kohler and Onan powered Gravely's, which don't share engine and transmission oil. But they're only using the synthetic oil in the engine and not the transmission.
IMO opinion, going with non-synthetic mulit-weight engine oils works just fine, for any Gravely application. Today's engine oils, with their advancements in properties and refining, put any oils from the 60's and 70's to shame. Living in Northern MI, I stick with 10W-30 during the Winter months - my work machines are in an unheated garage - and I'll often even tip my Kohler powered tractors backward, to send more oil toward the tranny/engine connection, for start-up purposes.
Even in the summer I'm often running with 10W-30; if the weather projections are for a very hot Summer/Fall, I'll switch to 10W-40. I have utilized straight weight oils in my Gravely's, but usually only with those machines using a little oil. If I lived in much warmer climates further South, I would entertain using straight weight oils more often. Standard 80-90W oil has worked well in my attachments - you just have to be careful to use the right type with any attachments possessing bronze gearing.