My 85 307d which was a 22' class A motorhome built on the 307d chassis with the 616 engine never had an oil cooler. I sold it at about 128k miles. While it was only driven the Europe, the loads what engine saw were way higher than anything a 240d will see. So, no it isn't necessary.
I suspect that the oil cooler installation may have been correlated with air conditioning.

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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