There's not a lot of difference in 20w engine oil and 80w gear oil *except* some extreme pressure additives. The viscosities are very similar - check out the website below for more info.
My CA's manual (original with the tractor in 1951) says 20w engine oil - that would be non-detergent engine oil. We've used 90w gear oil in the finals for all our AC's for years (30+) with no problems. Either way you should be fine.