P38A Is this oil safe for my car?

I've got a 4.0 P38 2000.

I happen to have a 25L barrel of 5W-30 fully synthetic which I bought for a 2009 Jaguar 3.0D that I'm now getting rid of, but would like to use the oil if I can

Two things concern me about this oil.

1. I have a slight weep from the rear main seal so will I regret going from 5w-40 to 5w-30?
2. This oil is fancy oil. It's labeled WSS-M2C934-B low SAPS fully synthetic which probably means lots of things, but one of them is low ash for cars with DPFs. Obviously irrelevant for the RR, but does this matter?

If point 2 is irrelevant, and point 1 might be a slight concern, could I buy some 5w-50 (maybe a semi-synthetic) and mix it. Or does oil not work this way? Another problem with optionis the price this price, 5w-50 seems to be really expensive and might as well just buy 5w-40 unless I can find a reasonable supply of it.

Any thoughts?

personally, if you have a leaking oil seal, then it will leak worse with a thinner oil. if you dont have any leaks, you might find some appear
bearing in mind this is a 1960s engine, land rover chose 10w40 semi/synth based on slight/marginal emissions improvements afforded by the thinner, slippery oil.
flat cam lifters don't like thin slippy oil, the oil of choice seems to be valvoline SR1 20w50 which has added zddp to help the lifters
its traditional mineral oil and not expensive

No good for the ancient (as in design) RV8 as the oil pump likes something to get hold of, not 'slippery' stuff.
Use a mineral 20w-50 preferably with a high zddp (zinc) content as it's a flat tappet engine.

nb. not sure if your car has a CAT in the exhaust, if so a high zinc content is not recommended.

norseman said:

No good for the ancient (as in design) RV8 as the oil pump likes something to get hold of, not 'slippery' stuff.
Use a mineral 20w-50 preferably with a high zddp (zinc) content as it's a flat tappet engine.

nb. not sure if your car has a CAT in the exhaust, if so a high zinc content is not recommended.

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It does.

Grrrrrr said:

It does.

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Thank you.
I'm a Classic man myself & therefore don't suffer that complication, though I stick to my 20w50 mineral recommendation for the OP

No 5w30 won't work, it'll run on it sure.. However it won't do it any good - 5w40 10w40 -20w50 10w60 will work.