Oil change info

So, according to Jaguar (images posted above by Jerry), an average of 15C…10W/30, or 10W/40 mineral oil, or even lower viscosity synthetic would be correct for your temperatures and usage.

Synthetics don’t break down under high temperature duress, anything like as quickly as mineral oils do, so the higher temperature are not an issue.
I suspect your driving pattern is relaxed cruising, not endurance racing, so the oil won’t get hot enough to need the higher viscosity.

I doubt that: it will conform to 1/1000" per 1" crank journal diameter.

Racing engines have sloppier bearing clearances and thicker oils to reduce friction.
Too thick an oil simply creates higher friction in standard clearances.

Film. Wedge. Flow. 15/20w 50 clearly has the advantage with the former 2. Little downside with the latter, especially if synthetic considered. Hot engines always a concern. Decades of use, Jag. recommendations, and good old common sense speak loudly. Perhaps most Jag owners simply graduated from plowing snow with cold engines with these cars. If and when you run your car hard with 0 or 5W whatever, kindly post results.

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Not really, no.

Cold starts = increase wear. Everyone knows that.

That isn’t killing an engine. Or, if it is, it’s a death that begins on day one and continues on for decades…with a strong probability that the car (or owner!) will be in the scrap heap long before engine death occurs :slight_smile:



You guys know the specifics much more than I do, so I just look at the manufacturer’s information and think … ‘if is is good enough for their warrant coverage, how bad can it be’ … acknowledging that these cars went out of warranty a very very long time ago, and for the ones we are still driving, they are still mostly having other issues, not oil wear issues.

Kind of like what I say about people (me included): if I am vertical, above the dirt, walking and talking when I go into a doctor’s office and when I leave … how bad can I be? Okay, don’t answer that. :smirk:


All else being =: Thicker oil apt to leak less, add to hot idle oil press and, last I checked, most major oil brands provide marginally more Zink in heaver oils along their given product lines. Older Jag.- you could do worst…

There is a few things can kill an engine, however, please outline your opinion on this statement (anyone may have a shot please)

90% of engine wear takes place within 30sec of cold start

(This is not my saying, but I tend to believe it may be close to accurate)

If that is the case, one may study what is the benefit or otherwise of running

the lowest viscosity (first number) that you can afford

I am not considering the second viscosity here, that should be as recommended by maker

The lower viscosity oil will lubricate the bearings measurably quicker on initial start up

This will reduce metal to metal contact

Go on folks, tear these arguments to pieces

( I do not always follow this advice, so we are talking theory thanks)

I don’t think of it as theory. I think it’s all true.

My position on the matter is that, although true, isn’t really all that important. :slight_smile:

No argument from me!

In my case we have have a 150k mile V12. In the years I’ve owned it is oiled with 10/40 in winter and 20/50 in summer. I can’t speak about older history but it is at least plausible that 20/50 was often used simply because 20/50 is so commonly suggested; for many it has been the “go to” choice.


Over the course of its life an unknown amount of wear has occurred and 90% of that wear has taken place during cold starts.

Ok, now what ?

If I begin using 10/40 exclusively how much benefit will I receive? Will it be a meaningful benefit? And how can I measure it? Will it change my enjoyment of the car?

If the engine needs an overhaul at 200k miles how can I confirm that it would’ve gone longer by using 10/40 exclusively? And, even then, how much longer?

I could certainly say that I “feel good” just knowing that less wear is taking place…even though I’m not likely to hear, see, smell, feel, or be able to measure the benefit.

If I remain a dogged traditionalist because it makes me “feel good” (there’d be no other reason) and continue using 20/50, what loss am I going to suffer via the additional wear? Is it meaningful? Measurable?

Seems to me that the answers to these questions would be nebulous and, if that’s the case, one “feel good” approach is every bit as good as the other ! :slight_smile:


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Good answer thanks

The recommendation is 0W-XX synthetic for all engines

I personally do not do this, as I believe it is overkill, costs too much (especially for Jag, huge oil volume)

However one either accepts, or does not accept that there is a directly measurable improvement in time to pressure lube bearings (and the rest) the lower the cold viscosity

I cannot testify its truth

I prefer to use a 10-50 in my rebuilt 4.2, but I am not fussy

I do buy a a 5W if its on special (for my other main vehicle which calls for 10-30)

It has very many miles on it so I go for a 10-40 usually. These engines are known to last over half a million miles, and have a more primitive design than an XK,

Never heard of an XJ motor going that long

I believe it’s more important how often you change the oil, than if you’re 10W too high or low. Friend of mine has a Mitsubishi, he only uses conventional oil, but changes it twice a year. He’s over 300,000 miles so far on original engine.

And (obviously, but not stated) the oil filter.

The first owner likely drove the car in the summer 20W/40 and 20W/50 temperature range (originally a New Jersey car and the lack of rust, along 61,000 miles over its first 25 years for 2,500 m/yr average).

I drove it about 65,000 over 15 years in the 20W/50 temperature range for 4,300 m/yr average.

So … I’m guessing … potentially 20W/50 for its life, which matches what the handbook says, albeit says it not real clearly.

With 90% of the wear being in the first 30 seconds, I propose that we never start the engines and save 90%+ of their wear.

What say you guys?

20w50 Castrol for me too. Easily 1,000,000 miles across 20 years of XJS driving, on V12s with 250k on the clock by the time rust, bad paint, worn interior, and general shabbiness retired them to the barn. Everything around the engine deteriorates, but that V12 just keeps going happily on 20W50. I drive them year ‘round and it gets cold here in NH. The bypass oil cooler keeps that oil hot even in the dead of winter.

So, high mileage and electric heater to keep the oil warm, excellent operating regime, no wonder your engines are lasting.

I honestly don’t think most owners follow that regime and as such should be more bothered about cold oil causing wear and contamination than hot protection.

This whole subject started really as a result of an owner suffering sticking valves leading to lost compression on a low mileage car.
Upon stripping the engine it was found to have considerable black sludge in the oil system. That black sludge is the result of the oil not being in it’s operating temperature, so not absorbing the contaminants.
Engines suffering from black sludge can suffer very sudden destruction due to constrained oil flow.
one minute all is well, next minute it’s game over and full engine rebuild time.

Panic reporting - valves expert opinion needed : sudden zero compression on cylinder 2, XJ6 1985 - XJ - Jag-lovers Forums

My advice to regular/occasional users is to change to a lower viscosity oil to aid engine lubrication and cleaning.

It really doesn’t bother me if posters either change, or not, but the question was asked, so a considered answer should be given.

And the information is read and appreciated. Thank you.

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Don’t sweat it TOO much, Deisel man- many “drink the coolaid” and make basic mistakes. 2 brief oil examples before I head to the barn: I was told about the recent Ram/ Stailantis try at a diesel V 6 light duty for some “trucks for yuppies” thing. Flawed- poor cranks. Also- they had to admit that their orig. “super thin” oil recommendations was no good. Retraction helped little. Positive example- One of the most successful 1990’s + V 8’s was Ford 4.6 L. Anyway- they started with a 5W 30 recommendation. Far from thick, By 93-94 they changed little, but specked 5W 20. Good news- most fleet (Police, taxi) and light truck guys drove em lots, but stayed with 5w 20 only for break-in. “up a grade” ea. 100k miles worked/ works!

I use Liqui Moly 10w60 full synthetic and change it every fall prior to winter storage…works for me. Ultimately you can’t go wrong with any good quality 10w40 or 20w50 as good oil is good oil.

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Yes, totally agree! You can’t just drive your car to the corner store and back.

But it doesn’t matter what weight oil you have, they heat up in the same amount of time. Not really sure how a 20w/50 oil takes more time to reach a specific temperature than a 10W/40 oil.

Thicker oil has a higher operating temperature than thinner oil…that’s the point of it being thicker.

Due to improved circulation the thinner oil may reach temperature more quickly, but at any given temperature the higher viscosity oil will not flow as well.
I did post up some technical data sheets earlier, perhaps you missed them.

Never heard of that. It simply has a higher viscosity.

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At a given temperature.

I have noticed there is a general lack of understanding/knowledge on this forum. Perhaps members might spend some time researching technical information.