Originally Posted By: Gokhan
What is all forgotten in this thread is that a conventional 5w30 shears to same viscosity as a 0W-20 after less than 1,000 miles.
If you are comparing a synthetic 0W20 to a conventional 5W30, then that is hardly a level playing field.
Also you make it sound like 5W30 always shears and that 0W20 never shears, of which I am not convinced.
But if you want a 5W30 and shearing may be a problem to you, then just make sure you select the correct 5W30 such as Castrol Magnatec 5W30 (was semi-synthetic but now full synthetic). There are ample used oil analysis posted here that show the Magnatec 5W30 is still in grade and holding viscosity well after 9,000 miles in Turbo engines.
Also if you want a 30 grade oil, and it never gets colder than 0 F (about -20 C) where you live, then just run a 10W30 for more shear stability and lower Noack volatility. Mobil 1 10W30 is a nice looking drop for this application, but even a conventional 10W30 should perform well.
But let's take a wider field of view and look for exceptions, as they can be quite interesting. I know in North America you can get almost any oil you want (except Penrite). But in Australia there are certain grades of oil they do not sell. Sure I can get a full GTL based Shell Helix Ultra 0W20 synthetic and GTX 20W50 conventional and Castrol Edge 5W30 A3/B4 which is BMW LL-01 & MB 229.5 approved, plus ample Valvoline SynPower of various weights and specs. So yes I have a good selection of good oils.
But, as far as I know, I have No Conventional 5W30 (ample good 5W30 as semi-synthetic or full synthetic). Also No Conventional 10W40 (ample good conventional 15W40, semi-synthetic or full synthetic 10W40, full synthetic 5W40 & 0W40).
What this tells me is that I should avoid conventional 5W30 and conventional 10W40, as these oils are being pushed too hard to meet modern API SN spec, and they are borderline oils. However if I want a conventional 30 or 40 grade oil, then I can easily buy numerous 10W30 or 15W40 mineral oils. Beyond that (lower W spec) you are in semi-synthetic or full synthetic territory.