Motorcycle Repair: 84 Gold Wing running hot, temperature gage, 20w50

Question
I purchased a '84 Gold Wing 1200 Interstate about a month ago.  Everything was running fine (except for a little electrical problem - another subject another day).  I changed the oil this week.  I used a Shell synthetic; based on someone's recommendation.  I don't know what the previous owner used.  The next time I rode it - about 40 miles at 4:00 P.M. ~ 87 deg temp around 70 to 75 MPH - the temperature gage went to 1/4 travel normally and then slowly worked up to about 3/4 travel on the dial and stayed there.  On the return trip; night about 70 deg - the gage went to 1/4 travel and slowly to 1/2.  The Honda folks said thermostat.  Not being convinced that was the problem and instead of shelling out $45 for a new one, I took out the old one and drove it around a little - at night ~75 deg about 75 MPH.  For the first 20 mins the gage was about 1/4 travel, then went slowly to 1/2 again until I starting riding about 60 MPH and it went just under 1/2 travel.  I do not recall this being the way it was before the oil change.  Surely it had nothing to do with the oil?  Right?  Any thoughts?  

Answer
Hi Mark.
 If this did start after the oil change, then I would go to the factory recommended oil or to an oil with a higher heat range, like a 20W50 or higher.  Something built for high revving engines should be good as well.

Let me know what you find.
Good luck.
FALCON