I picked up an old Ferris walk behind mower that has been sitting for years and years. Got it running and driving in no time. It even mows well. I want to change out the hydraulic oil but it says to only use 20w20 motor oil and I can’t find any info about a modern substitute. I can’t find either 20w20 or SAE 20 so maybe SAE 30 would be the next best thing? What would you use?