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On the pre-fss cars (before 98 or so) you just have to meet the API service classification. The API classification for a 91 is probably a lower classification than what is current, but it's difficult to find oil that is a lower classification than the current one, unless it's some pretty old oil that's been sitting around.
Again, just use an approprite viscosity for the anticipated temps for the next 3 to 6 months, however long the oil will be in there.
Gilly
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