Honda Motorcycle,1981 cm400c?

Just picked up an old honda M/C a cm400c (1981).
I'm getting a blow-by on the left front fork seal.
This 81 has 'air' added on top, not like the '80 that I also own.
So, do I need to replace the seals, or could I just go to a heavy fork oil; say a 30 or 40 weight. . . And does adding Air on top increase the chance of "blow-by" (or not)?
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And it looks like the 'fool' that owned it before used regular oil on the drive chain, not the sticky chain lube. (It's all throwing off)
So, can I just strip off the oil with a brake-cleaner and then after it's dry, add the chain-lube. Or will any cleaner I use damage the "o-rings" ?

---thanks