Thought this might be of interest ... someone on the GS forum wrote to Castrol with some questions about oil;
(A) Am I right in thinking that:
(i) Oils with two numbers, 10W-40 for instance, are known as multi-grades?
Yes, that's correct - roughly translated means it acts like a 10W oil when cold, and a 40 when at working temperature.
(ii) That multi-grades have been developed to run in an engine operating across a reasonably wide temperature range? For instance that the lower number of the two numbers means that, when the engine is cold, the oil behaves like a 10 weight but then, when the engine is hot, the selfsame oil behaves like a 40 weight oil?
Spot on - different engines will have different viscosity recommendations for different ambient temperatures though - always refer to the handbook for recommendations on specific applications.
(B) I have noticed that when I am in the south of France, where the weather hotter on average than it is here in the UK, the oils on sale generally have slightly different numbers than those I trip over in the garage at home. I guess this is because the southern French do not need the lower number to be so low (it's generally warmer) but do need the higher number to be higher (as the air and operating temperatures are generally warmer). Is my conclusion correct? I guess it is, simply because I see the numbers changing as I ride back up through France, with the oil in Calais, by and large, carrying the same numbers as UK oil. Again - correct, although you could argue that the thinner the oil when cold, the quicker the oil will circulate round the engine and build up a protective hydrodynamic oil film, giving faster protection to the engine components . would expect probably a 10W-40 / 15W-50 oil in France - but would still suggest a 10w-40 would be perfectly suitable for most applications up to 40º ambient.
© In the very hot summer we just enjoyed, would it be a good idea to move from a 10-40 oil to, say, a 15-40 or 20-50 oil? Or can I simply leave it? I see that the average day time temperatures in the summer, despite the hot weather, were still within the 10-40 band, even if the hottest part of the day were outside of the 10-40 oils recommended band. I guess one does not need to be exact and carry a thermometer and oil change around, ready for every hot spell?
Sort of - if you look in the BMW handbook it has a chart of different viscosity ratings for different ambient temperatures - the largest temperature range seems to be covered by a 15W-40 / 20W-50 engine oil. BMW tend to prefer basic mineral oils and I have heard very good reports about castrol GTX High Mileage 15W-40 which is what I normally recommend for the BMWs.
(D) As I understand it, it is OK to mix oils with different weights?
Yes, although it will obviously effect the viscosity of the oil.
Here is an extreme example of what I mean. Let's say I had 10-40 oil in the engine and it was half empty. I think it would be OK to fill up the missing half with, say, 15-50. From what I have read the result would be 12.50 - 45 and be quite OK. - more or less regarding viscosity - wouldn't expect any problems . I can imagine that some of the very extreme numbers may not mix too well? Is that right? However, as I am unlikely to come across some of the industry's more exotic oils at the average motor factors or garage, I guess I cannot worry about that too much?
In theory, you could mix a fully synthetic 0W-30 with a mineral 20W-50 ( if we look at the extremes ) and you'd end up with something like a 10W-40 semi synthetic - although we wouldn't really recommend you do this , but in theory it would be fine.
(E) Am I right in thinking that:
(i) All motor oils (mineral, semi-synthetic and fully synthetic) all start from the same crude oil base?
Yes, although Ester based synthetics are derived from mixing alcohols and acids - these are used as additives, and base oils in some high end synthetics, but most fully synthetic oils on the market will be PAO which are derived from mineral crude, which is why you can mix them ( motor oils generally available that is ) with no adverse effects.
(ii) That the synthetic oils simply carrying more additives than run of the mill mineral oil?
Sometimes, depends on the specification the oil needs to meet, but synthetic oils are tailored to do a more specific job so the additive package can be tailored to meet specific needs which could mean more additives are needed.
(iii) The 'fully synthetic' label is perhaps a bit of a misnomer? To me it implies it is fully artificial, suggesting it was made entirely in a laboratory, not from crude oil, then tweaked with man-made additives in the final process.
It depends - fully synthetics tend to be classed as lubricating fluids made by chemically reacting materials of a specific chemical composition to produce a compound with planned and predictable properties. think of them as very pure base stocks where the waxes and impurities have been completely removed by chemical process.
(iv) That semi-synthetic oil is simply a mixture of base mineral oil and fully-synthetic oil?
Yes - that is basically correct.
(F) If my motorcycle needed oil, I think is it OK to mix:
Yes, shouldn't be a problem although we would recommend trying to stick with the manufacturers recommendations where possible as this will give the best performance and protection for your engine.
(i) Mineral oil with semi-synthetic oil?
(ii) Semi-synthetic with fully synthetic?
(iii) Mineral with fully synthetic?
(iii) A mixture of all three?
I have no great desire to try just for the sake of it. I simply need to know that it would be OK.
Yes to all three above - although adding mineral to a synthetic will compromise the benefits, and may reduce the specification below that which is required for the vehicle, especially if a fully synthetic oil is required
(G) Am I right in thinking that it is OK to mix oil from any of the recognised decent manufacturers? Castrol with Valvoline or Esso with Morris, for instance.
Yes, if the oil is the same spec then no problem - if different spec then rules above apply - obviously, sticking with one brand is preferred, that way you are sure the additive package is not compromised by any slight differences between manufacturers.
(H) Whilst I think I understand the above, I am still confused by the other letters that seem to appear on the can. I think they apply to different countries' official ways to measure the quality of oil and / or refer to some additives that may be mixed in. Is that right? Do you have an idiot's guide? . http://www.carbibles.com/engineoil_bible.html some good general info here - not anything to do with Castrol, but useful none the less.
(I) Am I right in thinking that motorcycles with wet clutches should not be run on oils specifically designed for cars, as the oil may be too slippery, leading to clutch slip? It's not a problem, I guess, if the motorbike has a dry clutch?
General rule of thumb is yes - we get people saying they have used GTX Magnatec with no problems, but we are all bikers here and we all use bike specific oils as they protect the engine and gearbox and allow the clutch to perform properly. Car oils are designed for car engines and can cause clutch slip and early gear wear.
(J) Am I right in thinking that, as the BMW flat twin is not a particularly high revving engine (unlike say a jap 600, going up to perhaps 17,000 RPM) there is no great need or advantage in using anything better than decent semi-synthetic oil, once the engine is run in. BMW seem to recommend mineral oil for at least the basic running in period 600-1000 miles and, apparently, up to about the next six thousand miles (some dealers say 10,000).
I'd say that is a good rule of thumb - BMW engines tend to take a long time to run in ( 10K ) and if not done properly can tend to burn oil later on - BMW specify a basic mineral, although many dealers use a semi synthetic as a service grade.
(K) Is the old joke, "Any oil is better than no oil", near enough true?
Yes, but the "right oil" is always better than "any oil"
Hope the above helps
Kind regards
Andy Griffin
Castrol Technical Support
