Okay, why did Jim have to post the link to bobistheoilguy.com. Being a slightly obsesive guy, I have spent about 6 hours on that board, and there is a ton of information, including oil analysis feedback.
Based on the information I have seen there, I will add a few more comments/suggestions.
1) Mobil 1 tends to be blended on the thin side of the weight scale. So their 10w-30 tends to be closer to a 10w-20, and the 0w-40 with a cst @ 100 of 14.3 is on the borderline of a 30w oil. Given that I would feel entirely comfortable running the 0w-40 in all climates and not be worried about it being to thick.
2) Mobil 1 formulas vary depending on the contentent they are from. The sole exception to this is the 0w-40 which is a worlwide oil.
3) Castrol is now importing Syntec 0w-30 from Germany to the US and Canada. Unlike the Syntec which has been sold in the past in the US, this is not a Class III crude refined but a top grade Class IV PAO/Class V Ester stock oil. In addition this oil runs thick for a 0w-30 with a cst at 100 of 12.1 (compare that to the mobil 1 30w with a cst of 10.0). The talk on the lists is that this may be the best 30weight oil available in the US. BE AWARE THAT YOU HAVE TO LOOK CAREFULLY TO GET THE CORRECT FORMULA OF THIS. The proper syntec (as opposed to the garbage) will be 0w-30 with a red lable and on the back will say "Made in Germany."  
o not buy any other syntec besides this.
4) Amsoil is regarded as anything from snake oil to pretty good. Most of the oil lab testing doesn't show it to be anything spetacular; and
5) For those of you looking for heavier than Mobil 1 0w40 (which you shouldn't need), the new Redline 5w-40 with a cst of 15.1 at 100, looks to be very promissing according to the initial tests. This oil is made (to the best of my knowledge) from only Class V ester based stocks which are generally considered the best.
All of that said, the reason that most of the guys on the bobistheoilguy forum do oil testing (besides seeing how well is performs) is to see at what point the oil starts breaking down and should be changed. For the synthetics, it appears between 7500 and 15,000 miles is the proper interval. Factors that influence this are 1) oil capacity, the more the engine holds the futher between oil changes, 2) driving conditions, 3) quality of the oil, and 4) the condition of the engine.
There is also an interesting consensus concerning oil weight on that board, and it is essentially that as a motor gets older and starts to burn oil, it is okay to go to a heavier weight even though the manufacture may not recommend it.
Finally, on an off topic note, for those of you with late model Toyota cars (98-00) with the 4 cylinder or V-6 motor, be very careful. These motors apparently suffer from what is called black death, a buildup of sludge due to the gear drives breaking down the oil. If you have one of these cars it is recommended that you change mineral oil every 2500-3000 miles or synthetic every 5,000.
As for me, I have 8 quarts of Mobil 1 0w-40 waiting to go into the LC on Saturday. I am probably going to go to 7500 mile oil change intervals given that the oil should easily last until 10,000. I have considered running the Redline 5w-40 since 1) Redline is a local company and I can get it for $6.49/quart, and 2) I would only need one type of oil since my wife's BMW can use it (I can supposedly use the Mobil 0w-40 but I get concerned since it is on the thin side for 40w). I'm not using the Redline because I bought the Mobil already.
Cary
Based on the information I have seen there, I will add a few more comments/suggestions.
1) Mobil 1 tends to be blended on the thin side of the weight scale. So their 10w-30 tends to be closer to a 10w-20, and the 0w-40 with a cst @ 100 of 14.3 is on the borderline of a 30w oil. Given that I would feel entirely comfortable running the 0w-40 in all climates and not be worried about it being to thick.
2) Mobil 1 formulas vary depending on the contentent they are from. The sole exception to this is the 0w-40 which is a worlwide oil.
3) Castrol is now importing Syntec 0w-30 from Germany to the US and Canada. Unlike the Syntec which has been sold in the past in the US, this is not a Class III crude refined but a top grade Class IV PAO/Class V Ester stock oil. In addition this oil runs thick for a 0w-30 with a cst at 100 of 12.1 (compare that to the mobil 1 30w with a cst of 10.0). The talk on the lists is that this may be the best 30weight oil available in the US. BE AWARE THAT YOU HAVE TO LOOK CAREFULLY TO GET THE CORRECT FORMULA OF THIS. The proper syntec (as opposed to the garbage) will be 0w-30 with a red lable and on the back will say "Made in Germany."  
o not buy any other syntec besides this. 4) Amsoil is regarded as anything from snake oil to pretty good. Most of the oil lab testing doesn't show it to be anything spetacular; and
5) For those of you looking for heavier than Mobil 1 0w40 (which you shouldn't need), the new Redline 5w-40 with a cst of 15.1 at 100, looks to be very promissing according to the initial tests. This oil is made (to the best of my knowledge) from only Class V ester based stocks which are generally considered the best.
All of that said, the reason that most of the guys on the bobistheoilguy forum do oil testing (besides seeing how well is performs) is to see at what point the oil starts breaking down and should be changed. For the synthetics, it appears between 7500 and 15,000 miles is the proper interval. Factors that influence this are 1) oil capacity, the more the engine holds the futher between oil changes, 2) driving conditions, 3) quality of the oil, and 4) the condition of the engine.
There is also an interesting consensus concerning oil weight on that board, and it is essentially that as a motor gets older and starts to burn oil, it is okay to go to a heavier weight even though the manufacture may not recommend it.
Finally, on an off topic note, for those of you with late model Toyota cars (98-00) with the 4 cylinder or V-6 motor, be very careful. These motors apparently suffer from what is called black death, a buildup of sludge due to the gear drives breaking down the oil. If you have one of these cars it is recommended that you change mineral oil every 2500-3000 miles or synthetic every 5,000.
As for me, I have 8 quarts of Mobil 1 0w-40 waiting to go into the LC on Saturday. I am probably going to go to 7500 mile oil change intervals given that the oil should easily last until 10,000. I have considered running the Redline 5w-40 since 1) Redline is a local company and I can get it for $6.49/quart, and 2) I would only need one type of oil since my wife's BMW can use it (I can supposedly use the Mobil 0w-40 but I get concerned since it is on the thin side for 40w). I'm not using the Redline because I bought the Mobil already.
Cary
