Engine oil viscosity is a measure of how runny it is, thus indicating its ability to coat and protect metal bearing surfaces from rubbing together metal-to-metal. Oil viscosity is quantified by a number, usually from 0 (very thin) to 90 (very thick). But this viscosity is inversely proportional to temperature, so hot oil is runnier and less able to protect big gaps as found in older engine designs, and cold oil is heavy and sluggish, draining some engine power and again reducing protection. So a fixed-grade oil has to be thick enough to protect the engine at maximum temperature but this means it will be very thick at cold temperatures, making the engine very hard to start. Cold climate drivers sometimes have to resort to heating the engine with a paraffin heater before they can be started!
Many years ago, engine oil was blended to be ‘visco-static’, which meant it behaved as a thick oil when hot and a thin oil when cold. This allowed easier starting in cold weather, and good protection when hot.
The terminology will be familiar: a ’10W-40’ oil behaves as a thin 10-grade oil in winter weather and a thicker 40-grade oil when hot.
The Jaguar V12 engine and all 6 cylinder engines before the AJ6 engine of 1980’s vintage needed quite a thick oil – usually 20W-50 – partly because the engineering tolerances were not so close as they are today with modern manufacturing methods, and one needed a good thick oil to fill up those tolerances. As engine manufacture got better, one could use thinner and thinner oils that gave as good protection but much reduced drag. Rather like stirring a cake mix, it’s hard work churning up thick oil inside the engine and loses a fair amount of power.
The AJ6 and 16 engines are happy with 10-40 in UK climate, but a worn engine will benefit from 20-50. The late 6 litre V12 is better made than the earlier 5.3 so can use 10-40 oil, but the reduction in friction is tiny compared with the massive friction of 12 pistons so there’s no real advantage to be gained. Castrol offers a high quality 10-60 mineral oil and this is fine for the V12. The Jaguar manual shows 5W-50 oil as being satisfactory for the AJ16.
There is another important factor to consider in oils (amongst many). Oils’ second-order properties are graded, usually by the American Petroleum Institute’s API nomenclature. Without getting into the chemistry, the later in the alphabet the coding is, the better quality the oil. Early oils might be API SC and the XJS and XJ cars require a quality of API SG or better. 20-50 SG is not easy to find these days, although Halfords sell their 20-50 ‘Classic Oil’ at the time of writing, but this is only API SE – not really good enough for ‘our’ engines.
There is no point in putting very thin, exotic synthetic oils such as Mobil 0-10 in the XJ engines. The engine will be noisier and leak more.
There is no great harm in mixing fresh oil, so while all V12 engines will leave KWE with 20-50 in them topping up with 10-40, 10-50 or 10-60 is acceptable if 20-50 can’t be found.
KWE has its own 20-50 specially prepared.