I used transmission fluid when I did my forks, I've used it on the last 3sets I did.
Motor oil is not an option for forks, it's too heavy.
If your brake fluids are black after flushing them, the seals are probably deteriorating on the inside, as old as the Ascots are, the brake internals only lAst so long.
Time to overhaul the calipers/master cylinders, I'd bet practically everyone on this forum has overhauled the 2 sets, and it's a fairly easy thing to do.