Lubricant Basic Info

Classification by Viscosity

Single-grade Oils (e.g. SAE 30) :
For some applications, when the temperature ranges in use are not very wide, single-grade motor oil is satisfactory; for example, lawn mower engines, industrial applications, and vintage or classic cars.
SAE Viscosity Grades for Engine Oils1 — SAE J300 Dec 99
The actual viscosity grade of a lubricant is determined by the Society of Automotive Engineers, for example SAE-15W40 for a multigrade oil and SAE-40 for a monograde oil. The first number (15W) refers to the viscosity grade at low temperatures (W from winter), whereas the second number (40) refers to the viscosity grade at high temperature.
SAE Viscosity Grades for Engine Oils1 — SAE J300 Dec 99
1. Automotive Lubricant Viscosity Grades
Engine Oils – SAE J 300, Dec. 1999
SAE Low Temperature Viscosities High-Temperature Viscosities
Viscosity Grade 2. Cranking2 (mPa.s)
max at temp °C
3. Pumping3 (mPa.s)
max at temp °C
4. Kinematic (mm2/s)
at 100°C
5. High Shear Rate (mPa.s)
at 150°C, 10/s
min max min
0W 6200 at -35 60 000 at -40 3.8
5W 6600 at -30 60 000 at -35 3.8
10W 7000 at -25 60 000 at -30 4.1
15W 7000 at -20 60 000 at -25 5.6
20W 9500 at -15 60 000 at -20 5.6
25W 13 000 at -10 60 000 at -15 9.3
20 5.6 <9.3 2.6
30 9.3 <12.5 2.9
40 12.5 <16.3 6. 2.9
40 12.5 <16.3 7. 3.7
50 16.3 <21.9 3.7
60 21.9 <26.1 3.7
  1. 1. All values are critical specifications as defined by ASTM D3244
  2. 2. ASTM D5293
  3. 3. ASTM D4684. Note that the presence of any yield stress detectable by this method constitutes a failure regardless of viscosity.
  4. 4. ASTM D445
  5. 5. ASTM D4683, CEC L-36-A-90 (ASTM D 4741) or ASTM DS481
  6. 6. 0W-40, 5W-40 & 10W-40 grades
  7. 7. 15W-40, 20W-40, 25W-40 & 40 grades
Multi-grade Oils (e.g. 10W-40) :
TThe temperature range the oil is exposed to in most vehicles however is too wide for single-grade oils, ranging from cold temperatures in the winter when the vehicle is started up to hot operating temperatures in summer weather. That’s where multi-grade oils come in:
By nature the oil will have a high viscosity when cold and a lower viscosity at the engine's operating temperature. The problem here is that during cold-starts the thick oil needs longer to get to the necessary engine parts and do its job, whereas at higher operating temperatures it might get too thin and the coating film could break in some places.

To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers (VIIs) are added to the oil. These additives are used to make the oil a multi-grade motor oil (e.g. 10W-40). The idea is to cause the multi-grade oil to have the viscosity of the base grade when cold and the viscosity of the second grade when hot. This enables one type of oil to be generally used all year. In fact, when multi-grades were initially developed, they were frequently described as “all-season oil”.