as I was trying to convince myself to use a 5w30 in place of a 5w20 I was thinking no I shouldn't due to 5w30 being thicker but then it hit me. The difference between an allowable 9.3cst at temperature and say a 5w30 that runs at 10.5 cat is not that much of a difference when you consider the entire time an engine is warming up its running at 50, 40, 30, 20 and so on CST.... Am i missing something here? I mean if a car can survive years of "warming up" where the oil is much thicker with seemingly no problem than there really shouldn't be much difference at all between say a light for spec 5w30 and a thick for spec 5w20 right? I mean my car running 5w20 during short trips for the past 10 years must have spent countless hours running super thick oil with no problem... Tell me if I am missing something here? This isn't even taking into account that over an OCI a 5w30 could likely shear down a bit making it even thinner than when it started. I guess I really can't see the big deal with running a 5w30 in place of 5w20 even though my mechanic said i shouldn't and that the honda cars are designed for the thinner oil.