Basically, diesels have a bit of an issue running in their cylinder bores, causing prolonged high levels of internal friction together with imperfect piston ring 'bed-in'. These 2 things lead to less power/efficiency during the extended run-in period until the engine can be considered opened-up. Petrol engines dont have such an issue in this way as they start destroying themselves on day 1, having no nice diesel to lubricate the cylinder walls.
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