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.