Countax C600H Hdyrostatic What Type of Gearbox Oil is needed to top up?

Hi,
I have a 1991 Countax C600H Hydrostatic Garden Tractor. The plastic Gearbox oil fluid bottle is looking empty. What type of oil is needed to top up it back up? there is a rubber lid/top on top of the fluid bottle, does it need a complete oil change, or just a top up. It's never been changed since 1991 from new/. Is 10W30 motor oil ok, or is it something else? Thanks for any replies.