Dear Dipen,
Your message is posted some Two Months back.
I happened to join the forum recently; I thought I might as well take this opportunity to share my experiences on the subject.
First and foremost, there is no set/affirmed condition of temperature for automotive Engine oil or gear oil for that matter (in response to your statement “Indian (hot) condition.)
The selection of automotive engine oil for a particular vehicular application depends on various criteria, ambient temperature being an important one.
Mono grade oils like Viscosity grade 40 as mentioned by you conform to a particular temperature –viscosity range under SAE classification and fall under summer grade. Other grades under summer grades are SAE 20, 30, 50, 60 etc. Similarly, “W” denotes the grades which refer to winter grades and are the viscosity grades of oils which will maintain the flow of oil in winter conditions at the particular temperature range under which they fall. These grades are mainly 0W, 5W, and 10W, 15 W, 20 W and 25 W. The winter grade of oils classifies Low temperature viscosities i.e. cranking and pumping viscosities covering temperature ranges from -5 degrees Celsius to -40 Degrees Celsius.
Thus speaking in simpler and layman terms for winter grades, lower the ambient temperature, usage of lower winter grade will ensure that the oil will maintain its viscosity therefore flowability during cranking and will pump with ease through oil pump at the specified range of lower temperature without damaging the engine during cold start. But since the winter grade oils have a very low viscosity, as the temperature of the oil rises during operation, this grade will tend to become even less thinner thereby leading to rupture of the protective film and engine damage. To overcome this problem, the winter grades are blended in right proportion at manufacturing stage with the summer grades which conform to high temperature viscosity range thereby yielding the multigrade oil.
The following points may give a brief idea:
• A multigrade oil is combination of two viscosity grades a) a winter Grade b) a summer grade
• The winter grade is a low viscosity grade of oil classified from 0W to 25 W under SAE J 300 classification which determines the ability of each grade to operate/maintain flowability and pumpability at different temperatures, 0W being the lowest temperature range grade and 25 W the highest range grade classifying cranking temperatures from -30 degrees C for 0W to -5 Degrees for 25 W and pumping viscosity of 60000 cP from -40 Degrees C to -15 Degrees C for 25 W.
• Similarly the summer grades 20, 30, 40, 50 and 60 fall under a particular range of viscosity under 100 degrees Celsius.
• Thus the selection of multigrade oil will depend upon the lowest ambient temperature engine oil is likely to witness during cold cranking.
• In order to understand the effect in change of parameters on converting from 20W40 to 15W40 lets see the temp range under which they fall:
a. 20W : Cranking cP max at Temp: 4500 at -10 degrees C
b. 20W : Pumping cP with no yield stress at Temp= 60000 at -20
c. 15 W : Cranking cP max at Temp: 3500 at -15 degrees C
d. 15W : Pumping cP with no yield stress at Temp= 60000 at -25
• Thus by lowering the winter grade, we have actually molded the engine oil to operate effectively in colder conditions.
Coming back to the change in grade from 20W40 to 15 W 40…….in Indian hot conditions:
• Are u referring to usage of the MG engine oil in vehicles at places where mercury dips below -10 up to -15 Degrees C?
If the answer is yes, go ahead and immediately change to 15W40...u may avert a serious damage to your engine during cold start.
• Are u in an area where the mercury does not dip below -10 degrees?
Keep using 20W40.
• If you operate the vehicle in places where the engine oil is not subject to wide temperature variances, keep using the Mono grade i.e. 40 viscosity. But that doesn’t give leverage for the oil to maintain flowability under varied changes in ambient temperatures.
In fact the vehicles which ply under variable temperature conditions moving from different ambients in different seasons should make use of the multigrade oils to take care of factors as explained above.
And moreover as the winter grade of the oil keeps going lower, the price of the oil keeps going higher. Therefore if 20W40 is able to take care of the operational temperature range why pay more for 15W40 oil? And if the engine requires an oil to take care of sub zero temperatures during cold start, why change to monograde oil? Because the damage to the engine is going to be much more costly than the difference in cost of Mono grade and 15W40.
GOTCHA…..Thanx
Sandy
