As I heard from a good source.
Air does not blow fork seals though too much fork oil does. Air compresses (becomes smaller than its original volume under pressure) and fork oil does not. Fork oil under pressure has to go somewhere and the fork seal is usually the weak link. The general fork oil level is measured at 170mm from the top of the fork tube.
Drain as much of the old fork oil as possible. Fill the fork tube with new fork oil, 5w, 7.5w, 10w, 15w, etc. Use a tube marked at 170mm. Insert the marked tube in the fork tube and siphon the excess fork oil out. Do both sides. The whole process takes about 10 minutes. I make a simple Fork Oil Siphon though I usually give them away. It cost me about $3 and takes 10 minutes to make the FOS.
2005 KLR 685
2015 Yamaha Super Tenere ES, 5/23/2015
2012 Yamaha Super Tenere; Purchased 7/30/2011; Sold 5/23/2015