That bathtub on the 60 series Detroit is fiberglass.Plastic oil pans are pretty common. Even diesel have/had them depending on manufacturer.
Watched a chap one day clean a poorly crimped injector harness with Cat component cleaner (the old good stuff) then proceeded to solder the terminal end, well started anyways. The fumes from the half can of cleaner ignited, the flame danced for a moment as the look of shock appeared on his face. Then there was quite a loud whooshing sound followed by a bigger pop as the oil pan blew off those 10 little bolts holding it on.
11 gallons of 15-40 spreads out quick in the summer!
And no, the pan wasn’t salvageable.