Takes 2 mins to fill them with nitrogen for about $10 per side. It has Nothing to do with the shaft seals and oil, technically.
It barely takes anything to fill them. In fact, all the nitrogen would totally be gone by barely touching the valve stem.
You could even run oxygen in them BUT, Nitrogen will not heat up and expand like oxygen will, meaning that your suspension will stay much more consistent and not heat up as quick or as bad.. It also will be less likely to leak down, as the nitrogen molecules are larger than oxygen molecules. Nitrogen gas works the same way in your tires as well.
The other point is that oxygen is reactive and will corrode the inside of the resevior, where as nitrogen is not and will not. You could use any other dry inert gas Argon, Helium etc. You can not use dry air because dry air is not truly dry, the methods used to extract the moisture do not give a truly dry gas unlike distillation.
Just throwing that out there.
Snag these fellas, good deal.