Changing fork oil

[arnehulstein]

I think my front forks seem to have lost some pressure after they replaced the seals. This was done a while ago before the previous owner parked the bike. Now it dives easily and much easier than I would want it to. I know there are replacement springs that are stiffer, but I would like to see the effect of an oil change first. As I am not a featherweight myself, I thinking of changing the oil from 10W to 20W.

Can anyone tell me what to expect from this step up? Should I go for 20 or 15?
What is the best way to measure the 269cc that I need to pour into the fork leg?