when the hairs are burning off your legs it's too hot.  

but I use T6, your circumstances may differ.  

you might get one of these for the front oil port...



http://suzukisavage.com/cgi-bin/YaBB.pl?num=1361141824

I have head temperature gauge on my Savage, it is part of the Trail Tech Vapor digital speedo I installed.

The head temperature is higher than the oil temperature - but it might give you and idea what conditions can make the bike the hottest.  There are only a few times my bike has been approaching a 300 degree head temperature and makes me start to worry.  I don't know that head temperature becomes critical...I only know what makes it get near to 300.

All of the "near 300" degree events have occurred when temperatures were up in the 90-95 mark.  I don't really know if humidity has any effect on an air-cooled engine - but in this area it naturally gets humid when the temps are that hot.
1)  Sustained interstate travel at 65 mph and above can make the head temperatures climb near 300.
2)  Sitting in city traffic can make the temps climb.
3)  Very slow first gear travel under throttle can make the temperatures climb.

The hottest my bike has ever been was on a steep gravel road climb at the Dragon Run last year, that required 1st gear to climb the hill and  the speeds were so low that the air flow was not sufficient for cooling.  I didn't feel the need to pull over and let the bike cool as technically it probably wasn't overheated - but it was the hottest I had ever seen the cylinder head temperature as it got up around 305 degrees f.

I believe you will not have any issues as long as you avoid going really fast of really slow when the air temperature is above 90.  I have changed the gearing on my bike by adapting Kawasaki pulleys to reduce the engine rpm at interstate speeds - and my head temperature has dropped from 290 to 270 at 70 mph....and the bike is much more relaxed at highway speeds.

Dave        

On a Yam XT-225, I also installed the ‘Trail Tech Vapor digital speedo’,
for the primary purpose, of a accurate sped, the Tach, and oil temp.

Using the, ‘compression’ fitting, the top of the cyl head,
their were times it would hit 400.  Real Hot days.  
(Yea, once in a while here, that actually happens)  
Now that got me, real nervous,  
(As I have always believed that Oil, that got over 300, was toast)

So went to the junk yard, got a, car oil sender, pulled a, oil gauge,
and rigged it up. (checked it by using boiling water, it was right on)

Installed it in the, drain plug.
And when the, ‘compression ring’ sensor hit 400,
the car oil gauge said 275.
Most of the time it was between 230-240.

Made me fee a lot better.  

YMMV (As said hear often)

But I would suggest, measure the OIL temp, not the Spark plug.