Setting sag on a trials bike never truly works. We hear it a lot with all the Enduro guys buying trials bikes now. Tightening up the suspension is counter productive as this creates a stiff ride that bounces off obstacles and the bike does not get traction like it did. But you must have the correct springs for your weight. Running soft suspension is not the same thing as the springs sacked out with too much weight.
If you are going to the trouble of changing the oil you might as well do it right. Take them off the bike and flush them clean, then refill to the proper height. Which is in the owners manual.