When I go to oil supply sites and look for which oil they recommend for my AJ6, I'm surprised to see that they often recommend full synthetic, sometimes 0W40, sometimes 10W60.
I thought from various advices here and there that a semi synthetic 10W40 would better fit, not to say even 15W40 or 20W50 when the engine would be tired.
Would that that be due to more benefits from the sale of expensive full synth oils?

 

Follow the chart in your manual. Here is the one for the AJ16.

http://www.jagrepair.com/images/TSB/XJS/12-58.pdf

Use synthetic if you prefer the benefit of longer intervals between changes or if it makes you feel better inside using synthetic. You can argue points all day, but ultimately, the engine was designed before synthetics, so I'm not sure the benefits are what you might get out of a modern engine.

That being said, I've been using Mobil1 0w40 for a few years now. I used to switch between 15w50 in the summer and 0w40 in the winter, but I got lazy.

 

Thx for the Jaguar Technical Bulletin's link.
It deals with the latest engine genrations though: AJ16 and 6.0 that could then accept full synthetic low viscosity oils.
No mention of whether that would also fit with older engines...

 

As Vee said, the engines were designed and manufactured long before synthetic oils appeared. There's no evidence that using such oils has any benefit and they usually cost more, so why change type, or viscosity for that matter?

 

I think the simple answer is suggested in your comment, but as an inverse relationship.... that is, the engines and specified oils were indeed manufactured long before we had current oil technologies.

The oils and specifications/certifications from when these engines were designed have in many cases long since been obsoleted and new technologies offer enhanced efficiency, wear protection and OCI.

There's nothing wrong with sticking to the original spec, but there are now options for better that may even help the engine go longer without a rebuild.

.

 

But that's the point, there's no evidence that the oils do any such thing. The engines typically outlast the car with 'old spec' oils.

 

Indeed.

I can cite numerous examples of engines with over 200k miles (some owned by members of the Dwyer family) still running very sweetly, owned by people who gave no more thought to engine oil than I'd give to deciding what color socks to wear. And boneyards are full of old cars with engines having tons of service life left....but the rest of the car is worn out, as you say, or for various other reasons no longer worth keeping.

We'd need to take a dozen new XJSs, half with conventional oil, half with synthetic oil, drive and service each of them identically for 200k miles or so and then tear down all the engines for examination and measurement of wear before any conclusion could be drawn. A week's pay says the difference, if measurable, would be negligible.

Cheers
DD