MFG's usually spec oil. And gear oil does indeed do a better job lubricating. The problem is when you loose the seal, and all the fluid, the gearbox is usually trashed before you know it.
Many "old timers" never used oil. Just a light weight #0 or #00 grade grease. IT dont leak out like gear oil when you loose a seal. So no damage. Many really old cutters are still going strong and probably never had anything but grease in there.
IMO.....both work. Its up to you to decide but I dont think either decision is wrong.