THE INTERNATIONAL JAGUAR 'S'-TYPE REGISTER FORUM

I'm going to be lazy & request you read this thread from post # 20 onward & it's links. It covers the ZDDP issue. Feel free to ask any questions after that. The Mobil product above contains ZDDP in adequate dose plus other anti-wear agents.

I wondered how long it would take for the Zinc dialkyl dithiophosphate issue to be raised. I stayed clear of it because it confuses people in the extreme & there is so much nonsense written on the subject on the internet that I'm not surprised. For the average engine oil user sorting the wheat from the chaff is an understandable challenge.

https://www.jaguarforums.com/forum/mki- ... re-212846/

Thanks, Glyn. I've read all that and take it on board.

So, are people on this forum using XW-40 modern synthetic oils and getting >40psi oil pressure at 3,000rpm when hot?

I can't talk for people on this forum but I know of a number of XK engines running on XW-40's & lighter and achieving great oil pressure. Remember irrespective of the "X" W rating the engine is seeing an SAE 40 at operating temperature be it mineral or synthetic.

I'll let you know what my car does when I change. My car on an SAE 40 monograde at present has very healthy oil pressure. If anything higher than it should be. Of course absolute oil pressure is not that important as long as you have adequate pressure & flow to let the oil do it's job & hydrodynamic lubrication is achieved in the bearings.

Sudden change in oil pressure from what is normal for your engine is what matters as you experienced with the Penrite mineral classic 20W-50 due to VI Improver shear.

Glyn I have always wanted to know what the numbers on oil products actually mean. There are so many different grades? such as 0-40, 20-50, 5-30, 5-40, 10-40 and so one. What do the first and second numbers actually mean?

The viscosity of an oil is a measurement of its resistance to flow & is measured in Centistokes (cSt) ~ Kinematic Viscosity . There are two numbers that define the viscosity of a multigrade oil. The first number ends with the letter 'W', which stands for Winter. This measurement is related to how an oil flows when it is cold, such as at engine start-up. The second number is defined by how an oil flows at normal engine operating temperatures.

The smaller the number, the better it will flow. So a 5W-30 will flow easier than a 10W-30 at start-up temperatures and a 10W-30 will flow easier than a 10W-40 at normal engine operating temperatures. This is important, as engine oils naturally thicken as they cool and thin as they are heated. Thin, low viscosity oils flow easier to protect engine parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures. High VI (Viscosity Index) Synthetics thin less as they are heated & thicken less when they are cooled than their mineral counterparts.

The 40 in a 10W-40 simply means that the oil must fall within certain viscosity limits at 100°C. This is a fixed limit and all oils that end in 40 must achieve these limits. Likewise the 10W viscosity is measured at low temperature. Once again the lower the number, the thinner the oil: for example, a 30 oil is thinner than a 40 oil at 100°C.


Viscosity Classification standards for engine oils:

SAE 60 has been added since this with 100 deg C requirement of 21.9 <26.1 cSt. Modern oils have a High Shear Rate Viscosity [mPa.s] at 150 °C requirement. This is a complex issue/subject that will only confuse many.


Different viscosity classifications apply to different product applications. See below. e.g. An SAE 90 gear oil is of similar viscosity to an SAE 50 engine oil.

Thank you for that Glyn that is so easy to understand.
I always wondered!
I was told once that the XK engines oil ways are quite small compared to newer engines so a thinner oil ie 30 rather than 40 would be better. Is this the case to your knowledge?

Rob ~ I don't know if XK engines have smaller oil ways than is common with modern engines. It is more than possible. It is certainly why you would not want to put an SAE 60 into an engine with smaller oil ways as it would restrict flow for a given temperature.


Heavy viscosity 20W-50's is a bit of a British thing in part driven by Sir Alec Issigonis' Mini, 1100, 1800 "Landcrab" etc. where engine and transmission were common sump. The engine would have preferred lower viscosity oils & the transmission higher. Truth be known those old 20W-50's sheared out of grade very quickly due to the transmission gear trains chopping up the polymer VI improver.

The Jaguar history on oil viscosity requirements.


Monograde era S Type:


Multigrade era S Type:


Typical US EO Viscosity requirement of the era. 1957 Chev.

Great posts, Glyn. Thank you.

Thanks for posting that Glyn.

I found this article in this month's Motor Mechanic trade magazine that might be of interest. I was wondering if this would apply to oils claiming to reach the MB229.5 standard. Any thoughts on this Glyn?

Yes, excellent....