A double oil change on both engine and gearbox is a good idea.
Empty both of oil allowing them to drain thoroughly. Then refill will new clean oil in both.
Run the machine for long enough to warm up the gearbox and then do a second change.
The right grades of oil are in the manuals section on this site.
I do this when I’ve let something sit longer than I should have.
I’m told oil for equipment without forced lubrication and filtration like most mowers shouldn’t contain surfactants so junk collects in the bottom of the engine and/or gearbox. When used oil sits around it can break down and water from the atmosphere condenses inside the casings while the machine sits. Starting and using machines with unknown service history can churn all this stuff up making a mix perfect for destroying bearing surfaces.
An double oil change is good insurance.
