Originally Posted By: JAG
Most OTC 5w30 synthetics still have more polymeric VIIs than their 10w30 counterparts but the difference is not very big. I believe the best choice depends on the situation: engine, driving habits, climate, etc.
In theory a typical GP III 10w30 syn' may be slightly more shear stable than it's 5w30 counterpart but the trade off is usually a massively lower VI which to my way of thinking simply isn't worth it. If you need a heavier 30wt oil choose a heavier 5w30 or even 0w30 oil with a higher HTHSV rating and leave the 10w30 syn on the shelf. This way you won't be significantly raising the start-up viscosity (even in the summer) of the oil you're using which is what happens with every 10w30 syn'.
There has been a lot of progress in the past 10 years or so in motor oil quality, specifically with regard to VIIs. This has allowed for a steady increase in the viscosity indexes of motor oil. The 10w30 oil grade with it's typical 140 range VI is yesterdays oil grade; for all intend and purposes it is obsolete. For example Mobil 1 10w30, a very light 30wt oil with it's HTHSV of 3.0cP is heavier than Mobil 1 0W-40 and Pennzoil Ultra 5W-40 on start-up at temp's as high as 50F.
The two main reasons the 10w30 grade still exists is because it's cheaper to make than a 5w30 (higher margin) although it rarely ever sells at a discount to the 5w30 grade, and people continue to buy it largely out of ignorance.
Most OTC 5w30 synthetics still have more polymeric VIIs than their 10w30 counterparts but the difference is not very big. I believe the best choice depends on the situation: engine, driving habits, climate, etc.
In theory a typical GP III 10w30 syn' may be slightly more shear stable than it's 5w30 counterpart but the trade off is usually a massively lower VI which to my way of thinking simply isn't worth it. If you need a heavier 30wt oil choose a heavier 5w30 or even 0w30 oil with a higher HTHSV rating and leave the 10w30 syn on the shelf. This way you won't be significantly raising the start-up viscosity (even in the summer) of the oil you're using which is what happens with every 10w30 syn'.
There has been a lot of progress in the past 10 years or so in motor oil quality, specifically with regard to VIIs. This has allowed for a steady increase in the viscosity indexes of motor oil. The 10w30 oil grade with it's typical 140 range VI is yesterdays oil grade; for all intend and purposes it is obsolete. For example Mobil 1 10w30, a very light 30wt oil with it's HTHSV of 3.0cP is heavier than Mobil 1 0W-40 and Pennzoil Ultra 5W-40 on start-up at temp's as high as 50F.
The two main reasons the 10w30 grade still exists is because it's cheaper to make than a 5w30 (higher margin) although it rarely ever sells at a discount to the 5w30 grade, and people continue to buy it largely out of ignorance.
