Even the manufacturer doesn’t know what oil I should use. . . Short History: 1) Needed to find an oil that would flow faster a cold crank and reduce the astonishing amount of valve train clatter on my BMW Z4 (2.5i Sport), was referred to Total Quartz Energy 9000 0W30. Had been using the BMW OE 5W30 High Performance Synthetic till now (and I have the varnish and junk in the engine to prove it too). After the first startup, the valve train noise has practically disappeared. Absolutely, unbelievable. 2) I read on in this forum and learned many things. Several of you kind folks helped me with certain myths and misconceptions I thought to be true – and a great BIG Thank You for that. 3) We had established that ALL my cars required per their respective manuals, SL/EC ILSAC GF-3 ACEA A3/B3, the add BMW LL-01 for the Z4, and of course MB 229.5 for the Benz. I figured I would switch all the vehicles to 0W30 since ALL the manuals said that was one of the viscosities that may be used for operation ranging from -22F to +122F. I did just that. All cars currently run, 0W30 weight oil. Currently the 2 Jaguars are on GC 0W30, the BMW and the Benz are on Total Quartz Energy 9000 0W30. No problems yet, no complaints – extraordinary startups. 4) SITUATION #1: I recently misplaced the Jag's owner’s manual. Went to dealer and was given another one, here is what I found. I then went online to the Jaguar owner website (run by Jaguar) and did a bit of research. Keep in mind, this is for EXACTLY the SAME engine,4.2L Jaguar Supercharged, just different model year. 2006 “Jaguar recommends Castrol Oil Specification API SL/EC and ILSAC GF-3 .. . . . Your vehicle’s warranty may be invalidated by the use of improper oil. Failure to use an oil that’s meets API SL/EC and ILSAC GF-3 will cause excessive engine wear, a build-up of sludge and deposits and increased pollution and could lead to engine failure” 2007 (SAME AS 2006), except ADD for maximum fuel economy, 5W30 is recommended. 2008 “Jaguar recommends Castrol. An oil of 5W30 viscosity to Jaguar Specification WSS M2C913-B MUST BE USED in ALL vehicles. For top up purposes ONLY, where the WSS M2C913-B specification cannot be found, ACEA A3/B3 or ACEA A1/B1 may be used. 2009 (SAME AS 2008) 5) From what I’ve learned, in these forums, A1/B1 & A5/B5 API SM are Low Viscosity Oils and are NOT compatible with ALL engines. A3/B3/B4 API SL are high Viscosity Oils. To date it seems like only the “High Viscosity” A3/B3/B4 API SL/CF also meet the much more stringent BMW LL-01 and moreso the Mercedes Benz 229.51 Cert – which is a tough oil. 6) Well’ ignore the WSS stuff, obviously a Ford “thing”, but I am puzzled, that the manufacturer made a complete turnaround. From Euro Spec Hi Vis to Low Vis for the SAME EXACT ENGINE. They are basically telling me it’s OK to use the oil that would have caused engine damage a year earlier??? QUESTION: Currently using GC 0W30 in both Jaguars. I expect to change the BMW and the Benz to M1 0W40. These are ALL Euro Spec A3/B3/B4 SL/CF BMW LL-01 MB 229.5 etc…. which I have learned to be “HIGH” viscosity oils. Especially M1 0W40 (Which I might use in the Jaguars also due to that fact – VI index 185) I wouldn’t mind using one of those “Energy Saving” Like the new M1 Advanced Fuel Economy 0W30 – which meets : GM 6094M, GM 4718M (Corvette spec) ,Ford WSS-M2C929-A , Chrysler MS-6395 , ILSAC GF-4 energy conserving , API SM/CF , ACEA A1/B1 – although I see a proverbial WSS, what concerns me is the A1/B1 which is specifically what was forbidden to use a year earlier. I always thought that the higher the viscosity, the slipperier the oils is, the stronger the film is, and it “sticks” to components better which aides in cold startups, (along with the 0W)…. So is the new fad to run engines on low viscosity oils? or do I have this whole viscosity thing backwards? What are the pro’s and con’s of either?