When you buy a quantity of oil, probably the first thing you look for is the SAE grade. But what do these numbers represent; what do they mean? In this post we will explore the fundamentals of SAE J300, which is the standard under which these grades are defined and often a point of considerable confusion.
At its simplest, a multigrade oil is identified by two numbers:
1. The Winter grade, this is the number BEFORE the W
2. The SAE grade, this is the number AFTER the W
These numbers are not related.
Both numbers (grades) represent clearly defined ranges, outlined as follows:
Winter Grade
The Winter grade is determined by the oil's ability to meet the performance requirements of two tests:
1. CCS - Cold Cranking Simulator - This is designed to simulate the impact on the speed at which the engine can be turned over by the starter
2. MRV - Mini Rotary Viscometer - This is designed to simulate the oil's ability to make its way up the pick-up tube and be pumped
The limits for CCS are much lower than for MRV, so it is typically CCS that the oil fails, which then determines the Winter grade of the lubricant. So, for example, if an oil tested at 6,200cP at -30C, looking at SAE J300, we know it would not meet the performance requirements of the 0W-xx Winter designation, but we can see that it's below the limit of 6,600cP for the 5W-xx Winter designation, so the oil would be labelled as a 5W-xx.
*Multigrade oils are required to be labelled with the lowest Winter grade they meet* This means that an oil that meets the requirements for the 5W-xx designation cannot be labelled as a 10W-xx for example.
SAE Grade
The SAE grade is determined by the oil's viscosity at 100C and its HTHS viscosity at 150C. HTHS is only really a factor in grades where there is overlap, like 8, 12, 16 and 20.
SAE J300 from the Afton Specification Handbook:
When mixing oils of different viscosities together, you can use a viscosity calculator like the one provided by Widman:
So, for example, if we mixed equal parts of a 5W-20 with a KV100 of 8.6cSt with a 5W-40 with a KV100 of 15.2cSt, we get a KV100 of 11.34cSt, which, looking at J300, puts us right in the middle of the range for the SAE 30 grade.
But what about the Winter grade? That's more complicated, and cannot be calculated in the same manner. While the odds are, with both examples carrying the 5W-xx Winter grade, that the resultant mix is an SAE 5W-30, we cannot know that for sure. Different VII's, base oils and PPD's behave differently together and it's possible that it may not pass the 5W-xx Winter grade CCS and MRV testing.
Anomalies in the Winter grade are more likely with wider spreads, where one of the constituents of the mix is using a considerably heavier base oil, like say a 20W-50. So, for example, if you mixed a 0W-30 with a 20W-50, one might expect a 10W-40, but it may in fact be a 15W-40 or possibly even a 20W-40 (though unlikely), depending on how close to the 20W limit the 20W-50 was in the first place. Wax crystal formation, which is what drives the rapid increase in viscosity below freezing in non-PAO base oils, tempered by PPD's (Pour Point Depressants), is unpredictable in these mixing scenarios because the dose in each constituent is based on the blend for that specific product and a 4cSt GTL base oil for example, is going to require a different PPD dose than an 8cSt Group II.
At its simplest, a multigrade oil is identified by two numbers:
1. The Winter grade, this is the number BEFORE the W
2. The SAE grade, this is the number AFTER the W
These numbers are not related.
Both numbers (grades) represent clearly defined ranges, outlined as follows:
Winter Grade
The Winter grade is determined by the oil's ability to meet the performance requirements of two tests:
1. CCS - Cold Cranking Simulator - This is designed to simulate the impact on the speed at which the engine can be turned over by the starter
2. MRV - Mini Rotary Viscometer - This is designed to simulate the oil's ability to make its way up the pick-up tube and be pumped
The limits for CCS are much lower than for MRV, so it is typically CCS that the oil fails, which then determines the Winter grade of the lubricant. So, for example, if an oil tested at 6,200cP at -30C, looking at SAE J300, we know it would not meet the performance requirements of the 0W-xx Winter designation, but we can see that it's below the limit of 6,600cP for the 5W-xx Winter designation, so the oil would be labelled as a 5W-xx.
*Multigrade oils are required to be labelled with the lowest Winter grade they meet* This means that an oil that meets the requirements for the 5W-xx designation cannot be labelled as a 10W-xx for example.
SAE Grade
The SAE grade is determined by the oil's viscosity at 100C and its HTHS viscosity at 150C. HTHS is only really a factor in grades where there is overlap, like 8, 12, 16 and 20.
SAE J300 from the Afton Specification Handbook:
When mixing oils of different viscosities together, you can use a viscosity calculator like the one provided by Widman:
https://www.widman.biz/English/Calculators/Mixtures.html
to determine the SAE grade.So, for example, if we mixed equal parts of a 5W-20 with a KV100 of 8.6cSt with a 5W-40 with a KV100 of 15.2cSt, we get a KV100 of 11.34cSt, which, looking at J300, puts us right in the middle of the range for the SAE 30 grade.
But what about the Winter grade? That's more complicated, and cannot be calculated in the same manner. While the odds are, with both examples carrying the 5W-xx Winter grade, that the resultant mix is an SAE 5W-30, we cannot know that for sure. Different VII's, base oils and PPD's behave differently together and it's possible that it may not pass the 5W-xx Winter grade CCS and MRV testing.
Anomalies in the Winter grade are more likely with wider spreads, where one of the constituents of the mix is using a considerably heavier base oil, like say a 20W-50. So, for example, if you mixed a 0W-30 with a 20W-50, one might expect a 10W-40, but it may in fact be a 15W-40 or possibly even a 20W-40 (though unlikely), depending on how close to the 20W limit the 20W-50 was in the first place. Wax crystal formation, which is what drives the rapid increase in viscosity below freezing in non-PAO base oils, tempered by PPD's (Pour Point Depressants), is unpredictable in these mixing scenarios because the dose in each constituent is based on the blend for that specific product and a 4cSt GTL base oil for example, is going to require a different PPD dose than an 8cSt Group II.
But an SAE 30, for example... would they typically not qualify for a W rating at all? Or are they simply not tested? I guess I would think that with a somewhat decent quality base stock to begin with that an SAE 30 would meet some flow properties of a W rated oil.
