Originally Posted by Gokhan
The quality of the base oil depends mostly on the cold range (x in xW-y) and the spec to be met (SN, CK-4, A5/B5, VW 504.00, etc.), which puts a maximum limit on Noack. See the following charts.
Yes, when the spread gets larger, the VII content tends to increase. You can see that in the rise in the viscosity index VI.
I wouldn't read too much into the Briggs synthetic 5W-30 recommendation.
They were probably thinking of conventional vs. synthetic 5W-30. A conventional 10W-30 should perform similarly or better and a synthetic 10W-30 probably better than a synthetic 5W-30. In any case none performs anywhere close to a monograde SAE 30 unless the VII content is minimal.
A higher VI actually increases the engine wear. The reason is simple: HTHS viscosity at 150 °C is the set to be roughly the same for oils with different VI. Oil temperatures are usually in the range 100 - 120 °C. This means that an oil with a higher VI will actually run thinner at most operating conditions. However, you get better fuel economy with oils with a higher VI for the same reason -- the oil runs thinner for most operating conditions.
This is the reason why the Japanese OEMs recommend ultra-high-VI oils -- they are concerned about fuel economy and not so much about engine wear.
Thank you. That was helpful in better understanding of VI. It was perplexing as to Briggs manual would go from a mono grade down to a 5w30, bypassing 10w30 all together.
The highlighted
red part, I did not know/consider that but it makes sense.ðŸ‘
And yes, I agree with you... it's quite complicated stuff.
Originally Posted by Astro_Guy
Originally Posted by Silverado12
In the owners manual of my Briggs powered riding mower, it interestingly says that if You don't use a 30 straight grade to use 5W-30 full syn. Don't know why it doesn't mention 10W.
See the
B&S Oil Recommendations article and take note of the comments regarding 10w-30 oil. It appears that they are differentiating between synthetic 5w-30 and conventional 10w-30. They mention increased oil consumption when using 10w-30 at temperatures above 80F.
This implies that they are concerned about higher NOAK volatility, which is what Gokhan was getting at with his graphs.
I wasn't privy to the B&S literature but yes, it appears that is what they were thinking as Gokhan pointed out.
See what I get for wading into what I thought was an easily explainable question?...‚...‚ Oh well, at least I tried.