OILS... just a little poll

[phire]

Why do people use 15W40 as opposed to 10W40? Is 15W40 better for winter?

Actually, the colder the weather gets, the lower you want that first number to be. That's roughly the temperature in which it's still considered "pumpable" if I'm not mistaken. Modern multi-weight oils were designed to be thinner when cold so as to get oil to vital parts of the engine quicker. Let's say it was 5 degrees out and you had a straight 30 weight oil in your engine... You're really risking doing damage to your engine because a 30 weight oil is fairly thick and obviously the colder it gets the thicker oil will get... Anyways, rule of thumb is to base the oil you put into your engine around the temperatures you expect to be riding/driving in. So for summer, 20w50 wouldn't be a bad oil to use because it's thicker and won't thin out quite as much as say a 5 or 10w30 would... And when does the temperature drop below 20 degrees in the summer? Heck, when does it drop below 50?!?

Anyways, hope that was helpful and anyone more knowledgeable please feel free to correct where I may be wrong.

[mjm]

Why do people use 15W40 as opposed to 10W40? Is 15W40 better for winter?

Actually, the colder the weather gets, the lower you want that first number to be. That's roughly the temperature in which it's still considered "pumpable" if I'm not mistaken. ....

It is a viscosity ("thickness") measure - do not remember the scale off hand - there are several.  You set up a container with a clibrated hole and measure how long it takes a reference liquid to drip through, then compare other liquids to the reference liquid.  Lower number is thinner (drips through faster and will be easier to pump).  Higher number is thicker and will take longer to drip through.  The viscosity of many liquids, like oil, changes dramatically with the temperature.  In oil speak the first number is "winter" viscosity (thus 10W- or 5W etc) and the second number is thickness when hot - which used to relate to how well it lubricated in a hot engine.  Now it mostly tells you how fast it will leak out of a hot motor.  Other aspects of the API standard relate to how slippery and how much cushioning the oil will give under given conditions. Older engine designs need thicker oil at running temperature.  Newer ones do not.  My new MX5 uses 0W5 oil according to the manual.  Thinner oil also gives a bit better milage and easier starting.  I use 0W40 Mobile 1 in the GS500.  No idea what the dealer puts in the Miata -oil changes were included with the car.