My favourite source for credible information is the "Bob Is the Oil Guy" site, they have posted analyses and a great discussion forum. M1 0W-40, being Mobil's flagship oil, is a frequent conversation piece there. Search their archives and you will find loads of info.Originally posted by harrydog
The 0w40 oils interest me, but one thing bothers me. How can they obtain a viscosity range this wide without the use of viscosity index improvers? Some 10w30 synthetics use no VI improvers due to the synthetic base stock's natural properties, but I'm quite sure that even the best synthetics must use VI improvers to acheive this viscosity range. Am I wrong about this? If so, please show me where I can find credible information on the subject and not just heresay.
You might also look at the ACEA's official documentation on their various specifications. Currently ACEA A3 oils are the best oils out there, with the exception of specific manufacturer's brews - but even those start life as ACEA A3s in most cases (e.g., MB 229.5 spec, BMW long-life).
*Any* ACEA A3, MB 229.x (where x is 1-5), BMW long-life rated oil will protect your motor very well indeed and oil-related problems will be the least of your worries. The trick is finding one that Subaru accepts under warranty (the silly viscosity chart in the manual) at a reasonable price in the USA.
In any case don't fear the M1 0W-40 for *any* reason other than it's not an approved weight for warranty purposes. That's why I don't use it myself, preferring the M1 10W-30 (ACEA A5 only). Other than that, it's IMO a more robust oil than anything else available over the counter in the USA.