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Originally Posted by devarshi84 In India we have a Hot weather which touches 40-45 degrees celsius and hardly do we have a place which goes below 5 degrees in winter. Also, I read that a w30 oil mostly suffices for almost all engines since they operate at a much higher temperature (100+) than our 45degrees. Why then do we have oil grades upto w50? |
The viscosity index is not related to ambient temperature, it is the oil temperature. Oil viscosity changes with temperature. Hence the two ratings at 0ºC and 100ºC, which is approximately the operating temperature of the engine.
Shimla is very much part of India, and temperatures hit freezing. Would a 0Wxx oil be advisable there? Surely yes. For much of north India that doesn't see snow, 5Wxx will do fine. For southern India that barely sees a winter, 10Wxx or 20Wxx will do fine.
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Originally Posted by devarshi84 Why do oil makers sell us a 5w50 or a 20w50 oil when we just need a maximum viscosity of 10w30 or a 5w30? Numerous threads also pointed to the fact that a 20w50 oil shears much more faster since it has detergent and other additives which cannot flex like oil molecules. Will an air cooled engine like a high revving motorcycle engine require a w40 grade?
What I want the experts to tell the t-bhp community is what do we need different grades (irrespective of synthetic, semi synth, dino or the brands) when the max we might require is a 5w30? |
The hotter an oil gets, the less viscous it gets. The same oil at 80ºC might have a viscosity of 40, and at 100º 30. It's up to the manufacturer to determine, what viscosity is best at operating temperature.
Air cooled engines can form hotspots, with higher than average operating temperatures - V twins for example. Also, higher capacity/power engines run hotter. Hence Bajaj specified 20W50 for 180cc or bigger Pulsars, and 20W40 for the 150. Nevertheless, service centres have ended up using 20W40 on 180/200/220s and they run fine.
Higher viscosity oil offers more protection, but also requires more effort to pump through the engine - this robs some power. A thicker oil may also have extra resistance flowing through very thin oil channels, where a thinner oil would flow better. This is why Hero Motors switched from 20W40 to 10W30. It offers less resistance, thus helping fuel efficiency, however marginal.
At the cold start end of things, a 5W viscosity oil offers less protection than a 10W oil - but it flows sooners, gets around the engine quicker , ensuring quicker lubrication. A 10W may have better protection when it coats the metal - but until that oil flows and coats, there is no protection, so net effect is , your engine suffers more wear due to thicker oil.
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Originally Posted by devarshi84 Then why do most manufacturers use a 10w30 oil as a standard product?
What would happen if a 10w30 is used in Mercedez benz or Audi? Why will it not perform equal to a 5w50 (dino/ synthetic debate aside)? Since the viscosity is still enough and the oil can withstand the temps to not shear, what are the drawbacks of a 10w30 or let's say a 5w30 (or 5w40 if 5w30 is not available) for colder areas? compared to a thicker oil like 10w40/5w40/ 5w50? |
While the 5W50 may pump sooner on cold starts than a 10W, the 50 weight oil at operating temperature needs more effort to pump. Could the oil pump be over-stressed over a period of time, if thicker than recommended oil is used ? I suppose that's possible. Also remember the fuel efficiency angle - manufacturers are trying to use thinner oils to reduce pumping losses, while added friction modifiers and synthetic hydrocarbon chains to improve film strength to compensate for thinner film. That is a the significant factor behind the shift from xW50 to xW30 oils being recommended now.