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Diesels have a much higher compression ratio. This translates into an engine block that gets hotter. When something gets hot, it expands, when it gets cooler, it contracts. When it is hot, the tollerances between the moving parts is tighter, so you need more. Plus, because it is hotter, it is going to burn off more of the oil.

higher compression/temperature

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cos they often have bigger oil sumps?

they burn hotter