Maybe they are worried about the cold. The 0 refers to the low temp viscosity. In theory it should be just as viscous at high temps as say 15w40. I think in reality you may be better off with 5w40 or 10w40 unless you are in Alaska !! i-osprey, I would have thought that in the south you would have been using 10w40 or 15w40. Its not that cold there !
Often the oil companies dont bother testing every oil against every spec unless they want to get more $$ for it.
I know some of the latest little gassers from Korea use 5w50 oil all year as they have very small oil galleries in the motor and so need the oil to flow well at low temps.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving