Originally Posted By: The Critic
GM obviously knew that 0w30 will be synthetic, yet they chose to not allow it for all temps. Plus 0w30 would have given a fuel efficiency advantage as well.
Clearly there is a concern with the amount of VII in the oil, which will likely lead to more shearing and deposit build-up than a comparable 5W30.
I don't think it's as simple as that. Most manufacturers tend to specify only one grade of oil for their North American versions of the car. We don't see a lot of "xxw-xx is suitable above/below XX degrees F" anymore. Even when the manufacturers commonly had viscosity/temperature charts, there were some rather weird recommendations. We frequently saw 10w-40 treated as the red headed stepchild, probably for the VII issues you mentioned, which were more of an issue in the day. I know my old F-150, and even the Audi, had a couple strange recommendations; I can't quote them offhand, since the manuals aren't in front of me right now.
If the manufacturers specify one grade, barring any proprietary specifications or the occasional Japanese switch to 0w-20, they tend to specify something that can be had readily, and had readily in conventional, since some will balk at synthetic prices and make their own (probably worse) choice anyhow.
5w-30 fits the bill for GM, and 5w-20 does for Ford. I haven't seen Ford manuals from the past few years recommend 0w-20, either. Potential warranty issues aside (and that's another debate altogether), there should be no problem with a 0w-20 substituting for a 5w-20 or a 0w-30 substituting for a 5w-30 or 10w-30.
With the dexos1 specification, GM has chosen to go with the 5w-30 viscosity. There may be several reasons, including technical ones such as what you pointed out. Part of it, undoubtedly, is the path of least resistance for the consumers. Walmart and oil change places are quite likely to have M1, PP, and QSUD in 5w-30 with dexos1 approval. If dexos1 was a 0w-30 specification, I would expect more confusion. People would be using GC. People would be grumbling they couldn't find 0w-30. People would use 5w-30 anyway, and then probably just conventional, if this hypothetical dexos1 0w-30 was simply too hard to find and people didn't understand it would be at least semi-synthetic.
Perhaps down the road dexos1 will be expanded to include 0w-30 oils. I guess we'll have to see.
To the OP, there are 5w-30s that are thicker at operating temperatures than some 10w-30s, and 0w-30s that are thicker at operating temperatures than some 5w-30s and 10w-30s. Petro-Canada and Esso-Mobil are littered with examples. Anyone who tells you that 0w-30 is too thin for an engine that specifies a 30 weight is absolutely clueless, and would be the last person I'd listen to with respect to oil choices.
If your engine is specified for 5w-30 year round, use it year round with confidence. A 10w-30 isn't going to help you in summer; heck, it may be thinner than the 5w-30 of the same brand. The only beef I have with 0w-30 oils is the price of synthetics with respect to my oil change interval.
Even my ridiculously old school dad would have shaken his head at me for putting a 10w-30 in an engine specified for 5w-30. He was regularly scolding me with the phrase "too thick" long before this site came into being.
