Hi Robert,
Looking through my servicing notes, I did try SAE15 but went back to SAE10.
Have always put 250ml in as I don't have another bike with Marzocchi forks to confuse me.
I suspect the quantity is not that critical as long as there's enough oil to cover the damper system and not too much to overpressure the seals. The SAE 10 oil seems to give enough damping. You don't want it too firm - think of that Astralite!
Dave H