For the SP2, the preload adjust nut spins the adjust case, which is inserted into the adjusting plate. The adjusting plate rotates up and down the threads internal to the fork cap.
Here's a photo of the adjust case
And here's a photo the fork cap (upside down) showing the adjusting plate inside.
If the spring preload is very hard to adjust, the problem may center around the adjusting case movement.
Oil level would have no affect on this as oil level only comes into play as forks reach full compression.
The 489cc oil level for SP2 forks is with the oil lock piece in place.
If your forks are revalved, the company doing the work may remove the oil lock pieces.
In that case the oil level will need to be increased to prevent bottoming of the forks under heavy breaking.
The oil level in that case usually is around 120mm with springs out, or around 525cc.