Obviously they make 10w for a reason, I use it every summer and I'm from up north. You dont have to, but like they said it flows different until it heats up, which would be thicker until it does. I even use 20w 50 mixed in, or even strait, but of course I own a 92 5.7 that has lossed oil pressure in the past. New bearings, new pump, but works fine. I actually get more pressure using the thicker oils and it's still not half way up the guage. Some people have different needs, like newer cars vs. older. or whatever their reason.
