Personally I remove the rocker covers then top up to just below the push rod tubes with SAE 20, you also need to put some onto the cups on the end of the push rods where they meet the rockers. I usually do this once a week when we are boating. Normal multipurpose grease is fine in the cups….I have grease nipples on mine and give these a couple of squirts at the same time as topping the oil up. If you do it when the rocker covers are off you can check if the grease is going down the rocker shafts.
I would also invest in a good set of Whitworth combination spanners…you may also need some AF spanners if the fixings have been replaced at some point……Whit sockets are worth it too!….oh and when doing oil changes especially the first couple i would take the large side door off and give the sump a good mop out…..you may need to be prepared to replace the gasket on the door tho…and you will have to make one!….all part of the fun of vintage engines….I also take my oil strainer out of the sump and clean it but once you get the side door off you can see what you have….do you have a separate oil filter?
Cheers
Gareth