Just thought I'd point out that your owner's manual has a chart which clearly sets out what grades BMW _originally_ intended (with 198x oil technologies) the engine to use in which temperature environments. IIRC, UK summer & autumn temps fall in the 15w40 range for the 4-bangers, and 10w40 in our normal spring & winter temps (Dec - April).
As for which oil is best, that's a subject with a lot of opinion and little or no relevant data. IMO, use an oil with the appropriate API rating or higher and change at the specified intervals.
Mineral and blended oils will have less shear resistance etc., so you will not want to run them any longer than factory intervals. Fully synthetic may allow you to extend oil change intervals without additional wear, but to do so safely you need to do an oil analysis at your next oil change to determine wear metal content and oil performance, then continued analysis with a fully synthetic fill (and you're flushing the sump out, right?) at and after the recommended change interval to determine just when it actually starts to degrade in your motor.
e: equally, film strength should be higher with a top-quality modern synthetic, meaning you may in theory be able to run a less viscous (easier-flowing) oil to get superior cold-start protection etc etc - but to test this theory you need to pick the right weight and then test as above, which will cost you a packet and be a PITA. With the later sixes, you have the advantage of at least being able to see if the oil is BMW LL01 (LL04 for the newer cars?) approved, but for the M42 you are SOL.
If your motor burns oil and burns different amounts in different conditions, this is going to be tough to do (i.e., nearly pointless), as you can basically be replacing the entire sump capacity between filter changes and will not have a baseline figure for any oil you use.
Kos wrote:wrong,
a 5w40 is no lighter than a 10w40 at opperating temp. some engines use bit of oil as its in their nature to do so, some do it because its a tired engine. some do it because its shit oil.
10w40 is about right for this country and that engine, and a fully synthetic one. a 5W will help it warm up quicker, thats about it.
the only advise i'll give though is to use fully sythetic oil that will not break down
the questions is jon, how do you use your car, just commute, do you gove it some stick, do you do track days with it ?
have i been using the wrong grade all this time ?
