Yesterday I changed oil for 20w, as well as seals. Also added 1 cm "washers" to put a bit more preload on the springs. Haven't been able to test thoroughly, but first impression is that forks dive normally when braking, and the bike wobbles less when braking hard, then turning it. Also, front brakes seem to be workig better now - old oil was probably too thin (10w, but over 2 years old) - so no diving, bottoming out, guess it puts less weight on the front wheel.
On bumpy roads I don't feel things have gotten (much) worse - to my amazement.
Will test it more in the spring, see how it feels. So far, since I have adjustable preload forks (2001 model), the washers I put in can be neutralized by turning preload adjusters all the way out, while still leaving heavier oil in the forks for better damping. So far very pleased, will see how it goes.