Quote:
Originally Posted by AussieRacer
Put simply 10W-60 oil is thicker at a given high temperature than a 5W-30 oil is.
Now forgetting about turbos (cause I don't know enough about how oil interacts with them), at lets say 150 degrees (C) oil temp, using a 10W-60 oil will provide great protection to the engine by giving everything a nice think coating. Whereas at the same temp, a 5W-30 is giving a thinner coat of oil protection that is inherently going to cause slightly more wear, and "dirty" the oil sooner. So the case for 10W-60 looks good.
However if you're after every HP on offer, 10W-60 oil is a lot harder to push around the engine, and actually creates a slightly less slippery coating with surfaces, so an engine using 5W-30 with rev slightly quicker and potentially have some more horses (or part of a horse) under the bonnet than a similar engine using 10W-60 oil.
Now if BMW recommends 5W-30 oil, that can mean a few things:
1. The engine's oiling systems can't handle/don't like thick oil;
2. The 135i engine has lower oil temps than the BMW M cars;
3. The engine components are designed to wear correctly using 5W-30 oil; or
4. BMW doesn't think 135i owners are going to drive the cars hard enough to warrant full-time usage of a heavier oil.
What this all means, is pretty simple. If you plan on driving the car hard, change the 5W-30 oil more frequently, or use a heavier oil like 10W-60 (unless BMW specially recommends against using it for your car).
Hope that helps.
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I agree that the first 3 itmse listed here are good reasons not to use 10w60. It probably will not hurt the car, but it just might leave some parts without as much lubrication as was intended.