How much oil is needed for servicing? The manual calls for 5w light weight oil but doesn't specify how much. Anyone know?

Same amount you take out of the fork.

<140lbs, 40-45mm 38-42psi 5wt
140-180lbs, 35-40mm 40-45psi 5wt
>180lbs, 30-35mm 42-50psi 8wt
The oil is measured from the top of the leg with the slider fully compressed. Cycle it a couple of times and recheck to make sure there is no air below the damping valve. Use a little more oil to make the fork ramp up faster.

Chi, this link should provide you with some decent servicing tips, including oil level adjustment for the fork.

Thanks Rev and booyah for your help. My friend is trying to fix his Mag21 that's too stiff. He thinks he may have installed a valve the wrong way the first time he changed the oil (long time ago) but he isn't sure.

Sometimes to get good action on mags I had to add a "valve" to the damping piston with a drill. Booyahs link may help with this.
If your friend is a big guy using a lot of psi run the oil at 30mm and use 8 or 10wt. This will let you run a lower psi. These forks were known for seal stiction esp. at higher pressures.

He's a lightweight ... around 140 lbs. I think he might need to change some bushings too. Dunno.

He should be careful with that fork, the museum may want it back
It sounds like he need to increase the fluid flow thru the damping plate. There is a 2.5wt oil this might quicken his fork without having to make any permanent changes.

....... I'm hoping he'll register and start blurting out comments anytime now .....