Yes, there is a thermostat in the oil filter housing. Controlling temperatures are listed in a thread on here somewhere but I can't remember the figure. Search on 'oil thermostat'. I think it's just not hot enough for the thermostat to open. I don't think you have to worry about driving this time of year. Lots of these engines were sold without oil coolers. Driving across Texas in midsummer at 120mph with the AC on might be another issue.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13