It's steel. I've used two magnetic heaters on it before I installed the lower radiator hose heater. Was not impressed with their performance overall but they probably do make the oil warm. In no way did they approach the effectiveness of the coolant heater in the lower radiator hose.

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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