The manual recommends (from memory, someone correct me if I'm wrong) 20 weight winter (below 50 deg), 10 weight below 20 deg, 30 weight summer. That was before multi-vis oils were invented. There have been many acrimonious threads about this subject. A search will get you more than you want to know. I use 15-40 with an API SL or older rating. The new SM rating can be hard on flat tappet engines because it doesn't have much zinc in it. Some say it's OK if you don't have a hotrod engine. I'd rather not take the chance. You can buy aditive from GM if you can't find SL oil. I change my oil every fall. I put about 2000 miles on my 38 per year.

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Name your dog Naked so you can walk Naked in the park.