Typ 89 Oil temp gauge

[tihol_tiholov at sd27.bc.ca]

Haudi all, yesterday the oil temp gauge began bouncing on a cold-ish engine
(about 60-70 degrees C), then decided to stay high for awhile - around 150
or more.  Not much leakage, just a little sweat around the oil pressure
sender.  Previously the car had an exhaust pipe welded on and a week ago
new synthetic oil went into the engine.

Any ideas?  TIA

Greetings,
Tihol