On 1971 onward BSA/Triumph forks I use around 190cc to 210cc of fork oil per fork leg, depending how you like your forks to operate. You cannot change the seals without dismantelling the forks to some degree.
What I do in changing seals. drain fork oil, Jack up bike (if no centre stand), take out front wheel. Get a thin bar that you can place between the bottom of both fork legs between the studs. Undo the allen screw in the bottom of one fork slider. The slider will then come off and you can replace oil seal. Replace fork leg (remembering to replace allen screw) and do the same to the second fork leg. Put the wheel back in take the top caps off and fill the forks with oil , replace caps and away you go, till the next time.
I use either SAE30 or Fork oil, what ever I have at the time. Some use ATF. Don't forget to replace the o rings at the bottom/inside the fork sliders while they are apart.
Rich








