There are likely several correct answers to the OP's question.
I've had very good luck with Loctite 518 anaerobic sealer, one of the products Vic mentioned.
I first used it on a very leaky cylinder base. After lifting the cylinder as high as possible with the bottom ring still in the bore, I tore off the base gasket and cleaned the metal surfaces with laquer thinner.
A coat of 518 was somewhat awkwardly applied and that fixed the leak. It's been dry for about 30,000 miles now.
I also used it on the 1360 cases.
That sealed perfectly as well, but that engine had to come apart for a crank rebalance.
The 518 glues the cases together like cement, but it is low strength cement. On disassembly things were looking grim at first. It seemed the cases were welded together. After a few good wallops with a heavy rubber mallet, the 518 let go with a pop.
On reassembly , after scraping the faces clean, I used 518 again with the same result, a bone dry case joint.
Recently, for any surface needing a gasket, including the cylinder base I've used Cometics. They just work.
They are particularly helpful on the intakes flanges. These joints can leak and cause poor engine performance. Slight overtightening only makes things worse as it warps things, especially the carb flange. This can cause the slide to stick.
With the Cometic gasket the seal occurs with very little squeeze required. I put a drop of blue loctite on those nuts, since they are just barely snugged up.
I also had a complete set made for the BSA Super Rocket and let the A10 group know about this product. Now Cometic has all the A10 shapes on file, so they can reproduce them at any time, same as with the Vincent gaskets.
The response was pretty negative, a lot of " What's wrong with using paper gaskets?", " My bike doesn't leak a drop now" etc. Yeah right.
Oh well, horse successfully led to water...
Glen