Originally Posted By: mareakin
10w30 = 5w30 = 0w30 at operating temperature, right? Given a single brand, say, Pennzoil Platinum, the viscosity of oil should be the same!
right?
but reading this forum for a year, I pick up little hints that suggest otherwise, there is this sense that the *TRUE* viscosity goes like this:
10w30 > 5w30 > 0w30
any truth to this?
The problem is that there's a lot of variability allowed in what can be called a "30" or a "20." So even within brands, sometimes the 10w30 may be thicker than the 5w30 at operating temp. But ideally, they should all be very nearly the same at operating temp.
There's also a lingering feeling among a lot of people that a straight 30 will "always be thicker" than an Xw30 at high temperatures, but that's not always true either. Just for giggles once I pulled down the specs from Royal Purple's website, and their 10w30 is actually thicker than their SAE 30 at 100C.
The argument that a straight 30 weight with no VI improvers will be more reliable at high temp than an oil with a whole lot of VI improvers is also true... but in today's world the oil companies are making VII-less (or at least minimal VII and high-quality VII) 0w30, 5w30, and 10w30 oils from high viscosity index base stocks and by using pour-point depressants to get the "O" rather than relying solely on VIIs to get the "30."
So now that I've made a whole bunch of conflicting statements, I'll just say "it depends on the particular oils you're comparing" and walk quietly away...
Fortunately lots of oils have these specs published online, so you can look for yourself before buying.