Calculate suspension oil needed for a fork? I've got a non-standard 20" air fork (Mekkem SASO) that could use some maintenance, but there's zero info I can find from either the manufacturer or OEM on oil viscosity, volume, or anything else. I've watched many air fork service videos, so  I'm confident I can follow those procedures for disassembly and cleaning since things aren't likely to be very different from a hardware setup standpoint, but after I drain,  I'll be stuck trying to figure out how much oil to put back in. It's got 80mm of travel, so I'm wondering if I should just eyeball it (3ml?) of "standard" suspension fluid (5w?) or if there's a standard way to calculate what you need by measuring the volumes of the tubes and going from there. 

Given the lack of info on this item, I assume replacement parts will be difficult or impossible to acquire, so  I figure I should try to stay ahead of the wear to keep them around as long as possible. They don't get mountain-bike type use, just bad pavement and fairly tame single and doubletrack + gravel.
I can't find a specific source, but in my shop we use a chart from RockShox that can be applied to almost every front suspention. Here is a link