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Dick designed the engine. He should know. But everyone has an opinion and also facts based on their use of their Vipers. I have two and have used Mobil 1 10w-30, Mobil 1 15w-50, Mobil 0w-40, Pennzoil 0w-40, Pennzoil 5w-40. I do not race, I live in the snow belt and I do not drive the cars below 40F or with salt on the roads. High temps here get into the low 90’s but not often. In my temperature range and usage, I cannot tell the difference. I do an oil analysis at each oil change. I use Blackstone Labs. They can’t tell the difference either. Wear and contaminants are always in the same range and they always say I could have kept the oil in longer. Right now one has 5w-40, and the other has 15w-50 in it because I took it to a race shop for normal service and they said “that’s what we always use.” Based on my temperature range think Dick is correct about the w weight, 10 is fine since I am not starting the car below 40F and actually 15 is fine too since the garage never gets below 55F.