Just depends on how quick your M20 sumps, i presume you are talking about a M20 as you have a link to a BSA valve. If its dropping all the oil from the tank in a few weeks then the oil system wants looking at. This will be the pump and the non return valve in the base of the pump and the timing case. If the pump is badly worn then oil from the feed pipe of the tank slowly run through the pump round the return gears and past the loose ball at the base of the pump and into the sump.
If it takes months to fill the sump i would not worry about it. Don't forget in the past these bikes were used every day and they had no time to sump.
So your link shows an in line valve, these valves are good in fact i think they are the best i have on fitted to my 16H and it has been there for the last 10 years and has not been a bit of bother. You do not need the in line one as it will spoil your oil pipe line, you can buy one from the same seller that screws into the engine and replaces the original oil union.
If you fit this type you can hardly spot it and it will not alter your oil pipes, if you are lucky you just slide the bottom copper pipe further into the top rubber pipe you only have to do this for about 1/2". A word of caution if your oil tank is empty when you fit this bleed oil down to the sump valve by undoing the feed pipe from the valve and bleed oil from the tank to this so no air is in the pipe. When you buy the valve it has a set of instructions with it.
I have just built a M33 special long stroke engine (M20 flywheels) and fitted it to my XM20 i spent ages on the pump and oil system and when i first used it this year to my disapointment it lost around a pint of oil into the sump after around 3 weeks. I have run it through the summer and now its fine and has not lost any into the sump so all i can think off thet it has all bedded in.
