'89 150 Prov timing

[JKliverpool]

I recently purchased an '89 150 prov, while checking things over I found the max advance timing set at 20degrees. I changed it to the spec sheet 28 degrees, motor runs fine, but when talking to local yamaha dealer he said I should return it to 20 degrees because 28 is too much advance on a motor that old?? Is this true, I've never heard of motor age making a difference on timing.