I don't own a Rangerover but have a boat with a 1996 Ford 302 Windsor V8 engine, the marine version of this engine is known as the OMC Cobra engine.

In 1996 the road version of this engine would have a roller cam setup and use 5W30 synthetic oil. The OMC Cobra version is almost identical but has a hydraulic flat tappet valvetrain, marinas advise the OMC Cobra marine version should use straight 30 grade mineral oil.

I suspect the straight 30 oil recommendation is due to OMC being slow to change their advice. They are certainly much slower to change their engine / mechanical designs than is the case for road going vehicle manufacturers. The marine version has been around since the earliest road going Ford 302's which came out in the 60's, I think a multigrade like 20/50 is better than the straight 30 oil. But my question was/is would a modern synthetic oil be better than a mineral multigrade?

It's normally not recommended to switch to an oil with high detergents after running a no detergent or low detergent oil because the detergent could dislodge old sludge and cause problems but I rebuilt this engine last year and there won't be any sludge in it... So on that basis I would prefer to run a modern multigrade that has detergent to help prevent sludge build up.

This is where my oil concerns might be similar to oil concerns of P38 owners... Unlike the road going 1996 engines my marine 1996 engine has those flat tappets, which ZDDP protects, but modern synthetics don't contain ZDDP while a mineral 20/50 will probably contain at least some ZDDP though most manufacturers don't tell you how much. Though a mineral 20/50 oil won't contain much detergent.

I've tried to research what's the best option for my relatively newly rebuilt 302 with flat tappets on boating forums, car forums, oil company discussion boards, etc, got a headache looking into it.. Most of the boaters stick to the recommended straight 30 but (as said above) I think that's just following old advice that ought to have been updated.

Today I had a chat with Opie Oils who recommended Valvoline VR1 20W50 mineral 'race' oil. They said that although it's a mineral oil it has better sheer strength (retains it's 20/50 grade) better than most synthetics, the 20/50 grade is a good match for the old straight 30 grade, it has a lot of modern additives / detergents and has high levels of ZDDP. I bought 2 x 5L containers of the stuff from their online shop.

I'm not usually so anal / picky about choice of oil, for a car I'll usually just stick in whatever meets the manufacturer's recommended grade, choose a well known brand and not worry about it. But when you believe manuafacturer advice is 50 years out of date because newer / better oils have come along while they haven't moved with the times and your situation is a bit more unique because you don't need to worry about dragging up sludge in a newly rebuilt engine, it gets you thinking or at least it does me.

What oils do you run in your P38's and why?