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From an earlier thread:
>> The points made in VIKINGMAN's reply to you (below) seem to make some sense to me. Basically: (1) While being "in spec" [which is in itself a condition that is imprecise and subject to interpretation, as you noted] or listed by Saab is good, being "out of spec" or unlisted by Saab is not necessarily bad, and could even be better, given progress in technology.
I didn't see a reply from Vikingman, but I agree with you. I would be reluctant to let the "X" value (XwY) go too low in a sub-tropical area like the Deep South where I live.
>> And (2), viscosity is rated as XwY and in the context of synthetic oils, you want low X for starting and high Y for operation, especially for turbos, as you have noted. So it could be that with your choice of 15w40, you got the Y part "correct", but are suboptimal on X, and the proponents of X=5 or X=0 may in fact be "more correct". (This is not to suggest that 15w40 is a bad choice for LA, just possibly suboptimal.)
A 14w40 oil is approved by Saab for "summer" operation, but what exactly does that mean? Is it "summer" in Sweden or in Death Valley? I was told by Castrol many years ago that their 20w50 GTX (fossil) was approved for temps down to 20°F, which covers 99% of the weather experienced here. For that reason, I ran this grade year-round in my '83 and '84 C900T's and '86 9KT without any problems.
Later with my '91, I started out with M1 15w50 in summer and 10w30 in winter, changed at 5k intervals. Then in about '94 I changed to RP 15w40 year-round, and have had it tested several times with positive results.
In this climate I don't think a 15wY is too low, especially in non-winter conditions. Since RP does make a 10w40, I have that option, but the gurus at RP have told me they feel the 15w40 is fine for my application. I've always adhered to the rule of thumb that oils with a lower viscosity range (i.e., 10w30 vs. 10w40) are preferable to those with a wider range, mostly because the latter will be fortified with more VI's. In the past, this was not good, as they tended to break down more quickly than the base oil itself. But modern oils, especially synthetics, aren't as susceptible to this phenomenon. For that reason I'm not as concerned about it now, and maybe the next pale of oil I buy will be 10w40, still using it year-round.
>> I seem to be seeing more 5w- and 0w- on the shelves (TX) these days. Perhaps that is a clue?
Yes, there are more available, but this is mostly because auto manufacturers are specifying it. Some cars now specify 0wY, and most of the others are specifying 5wY. This is primarily to satisfy EPA fuel-consumption requirements, as a thinner oil will definitely be more fuel efficient, all other factors being equal. But I doubt the wisdom of using this thin oil in modern cars from a longevity standpoint, even if newer cars do have tighter tolerances than in the past. My '66 Corvette, for example, will always get 20w50, because compared to this Saab it is a bucket of loose bolts. The same goes for my Harley, which is fairly loose because of its air-cooled design, and it also gets a blend of 20w50 and Nitro-50 racing oil (50/50). It isn't a racer, but the racing oil is added as a fortification for those long, hot touring runs through TX or MS, or LA for that matter.
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