There are some ugly rumors going around that M1 0W-40 can't "make the grade" the way other oils do. Well, I'm here to stand up for an oil which needs no support, but can't defend itself because it has no fingers with which to type. It's not really a question of, "Did it stay in grade?" so much as, "How much did it thin out?" And even then, it needs to be taken in the context of the engine it's being used in. I took the liberty of sifting thru a few of the UOAs & came up with some interesting data. First of all, this oil is the single most popular oil for turbo owners, at least in terms of the data we have here. Of the 13 full syn UOAs I used, 4 were for M1 0W-40, or 31%. The other 6 oils were divided up with only 1 or 2 UOAs apiece. Unfortunately, this means the data's not very relevant for any of them yet. I also found 1 UOA for Castrol GTX in a turbo & 2 UOAs for M1 0W-40 in non-turbo applications, just for more statistically-irrelevant comparisons. I didn't look at UOAs which were less than 4k miles, since most of us agree that, even with the accelerated abuse a turbo gives oil, any full syn should survive for that short of a duration. I also didn't include any BobZoil numbers, simply because I couldn't find fixed VOA cSt numbers (only cSt ranges listed on the oils' data sheets). Most of the turbo engines were the VW 1.8T, with a few Volvos & one Subaru. Also, due to lab variances & the fact that VOA cSt numbers weren't available for the M1 5W-30, Amsoil 5W-30, & Amsoil 10W-30, I decided to use virgin cSt numbers pulled from manufacturers' data sheets for as many of the oils as I could. I used VOA cSt numbers for Lubro Moly, RedLine, & Castrol. M1 0W-40, 4 samples: Baseline cSt: 14.4 Avg mi: 4998 Avg cSt: 12.3 Avg change: 14.6% thinner Best mi: 4764 Best cSt: 12.8 Best change: 11.1% thinner Lubro Moly 5W-40, 1 sample: Baseline cSt: 14.0 Mi: 4100 cSt: 12.4 Change: 11.4% thinner RedLine 5W-40, 1 sample: Baseline cSt: 15.7 Mi: 5000 cSt: 14.6 Change: 7.0% thinner M1 5W-30, 2 samples: Baseline cSt: 9.7 Avg mi: 4463 Avg cSt: 9.8 Avg change: 1.0% thicker Amsoil 5W-30, 2 samples: Baseline cSt: 11.7 Avg mi: 4950 Avg cSt: 11.6 Avg change: 0.9% thinner Amsoil 10W-30, 2 samples: Baseline cSt: 11.9 Avg mi: 5565 Avg cSt: 12.0 Avg change: 0.8% thicker Amsoil 20W-50, 1 sample: Baseline cSt: 18.4 Mi: 10,000 cSt: 11.8 Change: 35.9% thinner Castrol GTX 5W-30, 1 sample: Baseline cSt: 10.6 Mi: 1350 cSt: 8.8 Change: 17.0% thinner M1 0W-40, non-turbo, 2 samples: Baseline cSt: 14.4 Avg mi: 8562 Avg cSt: 13.8 Avg change: 4.2% thinner So what do we know? That non-syn oils shouldn't be used in a turbo, & M1 0W-40 can go 10k miles easy in an NA engine. Maybe it's better to ask what we don't know. For example, these numbers don't show that both of the Amsoil 5W & 10W samples had 1 thining & 1 thickening, averaging to a nearly balanced result (the 5W-30 had a 17% variance between the 2 samples, while the 10W-30 had a 5% variance). In contrast, all of the M1 numbers were consistant (all the 0W-40s thinned, while the 5W-30s thickened). Does this mean that Amsoil's less predictable? Again, we don't have enough data. Is oxidizing better than shearing? I don't know. Here are my opinions for a turbocharged engine: 1) The "used" viscosity of M1 0W-40 is more desirable to me than "used" M1 5W-30. (Actually, more so than any of the M1 30wts, since they all start out lower than the 0W-40 ends up.) 2) The price & availability of the M1 0W-40 is more desirable to me than any of the "boutique" brands. 3) [There is no 3rd thing.] (Sorry, but I had to get Monty Python in here somehow ...) 4) Saying that a "thick" 30wt "stays in grade" but a "thin" 40wt doesn't, should now mean the same as saying "Blue oil is sweeter than pre-chewed eucalyptus leaves." Give me your opinions on all of this, & let me know where my analysis is flawed.