IIRC the one JC saw used M1 5W-30.
It could have been a metalurgy issue,
but more than likely a lack of EP additives in the lower viscosity M1's, particularly if it was a ILSAC GF-4 spec oil, which is more concerned with drag reduction within the oil and low phosphorous levels to minimise cat contamination.
OW-40 M1 should be fine, it's tailored to Euro engine specs. including light diesels, not US engines.
If you want to run a Mobil syn oil in a Land Rover diesel I'd use Delvac 1, but AFAIK there's nothing wrong with Penrite and the last time I spoke to someone that really knew oils from lots and lots of oil testing in a lot of different engines there's a lot right with it
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