It's a bit difficult to tell from the photo, but it looks like there might originally have been a letter, (now rubbed off), before the SSO - maybe an E for 'ESSO'?
This might be the modern equivalent:
https://www.exxonmobil.com/en/aviati...grease-shc-100
But, as Jerry says, I'm sure any modern bearing grease would do. I'd imagine lubrication technology has advanced in leaps and bounds since the days when British bikes ruled the road.