I've got my factory original 1978 Chevy 250 with less than 45k miles on it, with no major changes yet. This is a street car and driver. I've relied on the manufacturer recommendations and based on the climate of midwest, I have been filling it with synthetic 10w-30 every year. I don't drive it when it is below freezing here, and our summers are generally mild with only the occasional day in the high '80s or low '90s. According to the chart in the manual, it can use 10w-30, 10w-40, 20w-20, 20w-40, and 20w-50. With 5w-30 only if I used it during winter and the cooler season before changing it.
But newer cars I've owned have just recommended 5w-30 and that's it. This obviously isn't the same 5w-30 from 1978, so I don't know if this chart still applies these days? Is 5w-30 still too thin to run in an engine of this vintage in temperatures over 60 degrees anymore?
