SL is a pretty old specification (2001) and a pretty minimal standard at that. The later SM and SN standards imply SL, and those actually specify some useful properties, such as improved deposit and oxidation control. So you can't go far wrong with a lubricant that is compliant with the newer SM and SL standards. Mower manufacturers sometimes sell and recommend their own brand of lubricant for the Hydros, some of these are synthetic. I think all of them feature a robust anti-wear additive package, Some of them are pricey.
So the way I see it, there are 3 groups of solutions:
1) Purpose build OEM hydro oils, based on some 15w50 or 20w50 starting point. Some of them are group 3, some true synthetics.
Could be rebranded motor oils w extra additives.
They all feature boosted anti-wear, anti, oxidant and anti sludge additives. E.g.:
Exmark
Ariens 15w50 #00057100 - $15 per quart. Synthetic. Claims double service intervals
Stens Shields 15w50 synthetic: Similar, but cheaper ( $10-ish per quart. )
2) Purpose built 3rd party lubes for hydros:
Triax Zero Turn 20W-50 Full Synthetic -10-14$ per quart
Amsoil AHF 20w50 synthetic base, hi zinc. Coudl be group 3 based, not disclosed what base they use, $14 / qt
3) High quality motor or bike oils matching the manufacturer's listed requirements
Mobile synthetic 20W50 or 15W50.
Castrol 10W50 or 20W50 motorcycle oil Redline powersports 20w50 or 15w50 motor oil ( $50 per gallon)
It is hard to know which one is the "best" as we typ[ically do not have sufficient info on the formulation,
and we are making educated guesses of the "useful" / "not useful" ingredients or properties.
Racing oils have maxed out wear additives, but may not have enough acid neutralizers, as they are intended for short-time use. The service interval here is extra long so.. no. I am not sure if acid formation is even a concern for a hydro, where there is no combustion. When in doubt, be extra cautious.
Motorcycle oils sometimes have some extra friction modifiers for wet clutches, I am unsure if that is harmful for the hydro use. I go by: If motorcycle oils were best choice, it would make sense for Hydrogear to call those out. And the factory fill is NOT a motorcycle oil. So when in doubt...
In the final analysis, I am going with the Ariens 15W50 synthetic. I think Amsoil may be a good choice as well, I just dislike their marketing hype.
With the Ariens/Gravely product, at least I have some assurance that someone in the possession of all the requirements and specs
took a look at the formulation and approved it.
On the motor oil for the Kawi, I am trying the redline 15W50, what the heck. I always wanted to try it.