I've used 15w40 in my m103. I read a post on here talking about zddp additives and how they're not present in modern oil. Evidently our engines like this and well the diesels have it?
On the topic of intervals. I would change it and sent it off to be analysed. I havent done it yet for mine but a friend recommends it. They'll tell you whats in it and if you can go longer than you currently do and whatnot.
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