Loctite 5910, or 5920, Halfords sell the 5910, £5.45 a tube, or Dow Corning 728, not sure if you can get this outside the forces though ?
You don't need any on the cap seal, just around the ends where the tang of the r/c gasket slots in(if cometic), and all around the half moon, just let the 5910 go off for 5 minutes first so it skins over, but make sure it doesn't go over 15 minutes as it start setting fully, though full cure is 24 hrs.
I advise you to take off the front caps/bearing caps and put sealant between the cap and head, as many leaks from here get blamed on the r/c gasket, it WILL leak from here if not sealed.
Tighten the r/cover ONLY to the specified torque, NO more as this can distort the cover and lead to leaks.
tabetha