I know this is a hot topic and I have my favorites but I'm also not familiar with any of the perhaps best options as far as viscosity and needed additives for a 100,000+ Jag 4.0 engine.
I'm a Valvoline guy and run VR1 20W50 Conventional in my Harley.
I would tend to want to run Valvoline 5W30 or 5W40 perhaps maxlife or high mileage in a semi synthetic in my car. Any reason one of these wouldn't be a good choice in my 03 XJ8? How about a 10W30 or 10W40?
Would a High zinc race oil be better for the chain tensioners?
Filter recommendations?

I tend to change my oil often. More than necessary. Would a conventional be better/ok?
I've never been sold on extended OCIs and synthetic oil.

I absolutely despise M1 and Amsoil and won't use them. They're only best when it comes to marketing hype.

 

A lot depends on where you live (fill in the data with location). Moderately cold in VA and really hot in summer, I ran 5w40 full synthetic (I like Valvoline based on a study someone - here - did years ago; but Mobil ! is as good and often cheaper); switching to 5w30 for winter. But, there must be a trillion oil threads on this forum. . . .

 

Yeah, I read thru some of the threads.
I just know the M1V-TWIN, popular with a lot of Harley owners, doesn't hold up for crap in those air cooled v-twins compared to something like VR1 20W50 Conventional, which is one of the best oils you can run in a Harley according to UOAs done.
The M1 sheared then thickened to scary levels. And the VR1 is half the price.
When run at the same temps of 300°F, The VR1 lost a mere 3% of it's viscosity. Unfortunately they don't make it in a 5W or I'd run it in a hearbeat. They do make it in a 10W. Not sure about a 10W in the winter here in IA tho where temps can dip to -30 and colder.
I'm going the safe route with a Valvoline 5W30 maxlife semi synthetic.

 

if you want viscosity retention use an ACEA A3 or C3 rated oil instead of ilsac gf6

 

I just stuck with my trusty Valvoline Maxlife semi synthetic. 3qts 10W40 and 5qts 5W30. Should make for a nice 7W34 I'll be comfortable with once the weather turns colder as well!
I had the 10W40 here already so what the hell.

 

Jaguar recommends Castrol 10w-40 for the temperature range I drive my car in, so that’s what I use. However many brands and viscosities of oil there are on earth is how many different opinions there are on which is best. I’ve always used manufacturer recommended oils and maintenance intervals no matter what vehicle I’m driving, and I’ve always gotten along fine.

 

I agree run the recommended weight but that's as far as I go with manufacturer's recommendations. The do have ulterior motives recommending brands and even type. Extended OCIs have been pushed by the EPA as well.
Case in point is Harley Davidson and motorcycle oil. Why anyone would pay the price of motorcycle oil for a motorcycle that doesn't share a sump is beyond me. It lacks friction modifiers beneficial to engines because it has to work in wet clutches. Motorcycle oil is for motorcycles that the engine, tranny, and wet clutch all use the same oil. Harley's do not and a good high performance race oil with the same high zinc/phosporous levels AND friction modifiers is a much better choice for Harley engines and for less $$$.
So I take manufacturers recommendations with a grain of salt for anything but weight.

 

Some oil are better than others . I have seen friction tests for many Engine oil liqui moly did better than Mobil 1 and Castrol bizol was great too I'm using bizol full synthetic 10w40 good oil . I used 20w50 at summer nothing wrong but I can feel the engine doing more work to move parts . That's why formula 1 use lightest oil during racing because makes parts move faster but it cost more metal friction
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Everyone has a different preference of oil brand and oil weight. The most important thing is keeping it changed regularly and frequently checking the oil level. If the oil is clean and at level you should be good, the vast majority of premature engine wear comes from oil starvation or dirty oil that clogs passages and no longer lubricates as intended.

 

Been running Pennzoil 5W30 Platinum Full Synthetic for a couple years now, really like it.

Here's Jag's official oil chart.