I just bought an 83 300SD. Car runs fine...got 32mpg with first tank. With outside temps of 70-80f, I run 60-65c (140-150f), unless I go 80 MPH, then I get up to 80c (176f). I read the OEM Tstat opens at 185f...could be I'm stuck open or somebody put a 150f Tstat in it.
I'll pull and test Tstat...probably put new one in.

Does this seem low to diesel drivers?

Do diesels run cooler than gassers?

What temps do other dieselers run?