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There shouldn't be any rough parts in an M Engine, these are ultra precision made pieces of equipment, the purpose of a running in oil is that it is mineral based with zero additives, it is these additives (found in semi synthetic and fully synthetic oils) that are designed to prevent wear which you don't want in a running in (mineral) oil as this prevents the engine from bedding in properly. If you ran fully synthetic from the get go you'd end up with an engine that won't bed in properly resulting in poor piston ring sealing and increased blow by resulting in oil burn and power losses.
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