Just looked up the topic in my shop manual. 615 engines only had oil coolers if it had AC. 616 had oil coolers standard until 1978. After 1978 oil coolers only installed on 616 if they had AC. 617's always had oil coolers.

In looking at the diagram in the shop manual, it looks to me as though once the thermostat opens, it permits oil to flow both up into the filter and out to the cooler. It states "In the end position (with the thermostat open) the direct flow th comination fileter elemens is closed escept for a given quantity of oil. this oil quantity is enough to guaranett lubrication of engine at low outside temperatures, when the through-flow in oil cooler is considerable detained by the now viscous oil. The major oil quantity arrives at air oil cooler where it is cooled and then flows back to oil filter housing the through oilter filter housing from outside in"

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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