Did this just suddenly come to light on your bike, it was OK/good previously?
Before you do anything rash and you already have (changed fork oil viscosity) retorque all your front-end bolts as per sequence, guidelines can be found on this site. Double-check your wheel alignment and I stress double-check your wheel alignment before riding.
Tim, start with the simplest and easiest to fix first, tire pressures.
Set your tire pressures cold and go ride the bike, take a pressure gauge and pump with you change the pressures no more the 2 psi at a time and document your results.
Before you ride and after retorquing all bolts check your disc runout. Listen to Clem about tightening the bolt on the front axle, I chased a similar gremlin for a long long time and it was Clem's knowledge that came to my rescue, and at the time I had been a Laverda owner for more than 40 years. You can teach an old dog new tricks, Clem knows stuff.
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