Fuel mix for both gas and oil tanks, don't forget to gently blow out the inline filters in both tanks with compressed air. This should be done once a month for light use, once a week for heavy use.
If it's an old saw that has been sitting for a long time and has varnish in the tanks, use B12 Chemtool to clean out the varnish, then a once over with fuel mix. check the fuel and oil lines for cracking and replace them if they're brittle.
Good luck and work safe.
jomoco