All of the oil charts I have seen end at 86F. all 40W and 50W oils just have an arrow past 86F. My question is, at what temp is one supposed to switch from 10 or 15-40 to 20-50? it seems to be too cold in the summer mornings in CO for 20-50, but alot of people have told me to use that in the summer, which is fine when it is 90F but not so good at 60F.

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Will
94 E420
82 300DT
77 F250 4x4 429 Cobra Jet
69 Thunderbird 429 Thunder Jet
92 Mitsubishi Galant VR-4

15w-40 or synthetic 5w-40 are fine year round.

Check the owners manual and select an oil that covers your expected ambient temp range. The chart I have handy shows 15W-40 or 15W-50 is OK from about 23F up. As I recall, that agrees with the one for my '83.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

I also use 5W40 synthetic year around in CO. My owner's manual does not indicate an upper temperature limit for xW40 oils. I wouldn't consider 20W50 unless I was in an area with consistently high temperatures.

I have used 5w40 year round for years. MB now has switched to 5w40 year round on all the new cars.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200