Std 620 fork springs themselves are not too soft. The problem is that they are progressive springs - more suitable to road use. Because they are progressive you lose too much fork travel initially through sag. It is better to fit single rate springs.

If you go for heavier oil beware that heavier oils are more prone to cavitation i.e filling up with bubbles which, can actually degrade damping performance. It's always better to get damping valves orifices modified to increase the damping when using a lighter oil.

I talked to a few suspension houses about doing some spare 620 forks I have and the general recommendation was just to service the forks, go for a single rate spring and std oil.