Toyota Repair: Toyota Corolla 94 oil, toyota corolla, happy new year

Question
QUESTION: Hello Ted,

I live in Fort Lauderdale, FL (80 degrees temperature) and have a Corolla '94, 1.6 liters, Japan-made, 4-door sedan with 120k miles. The car is in excellent condition, b/c I don't drive a lot, and always drive respecting speed limit, do the maintenance as scheduled, and treat it like a princess.

My problem is that I have asked several people what the thickness for engine oil should I use for my car, and I have had many different answers.

Generally people tell to use 40 after 100k miles, but someone else told me that for my model, I should always use 10/30 no matter what the mileage. Some other people tell me to use a mix, and gradually add thicker oil as more miles are put in the car.

I just stopped controlling what oil is put in the car anymore. When I take it for maintenance, I let them put whatever they have. The thing is that I've noticed lately, the Hot/Cold indicator on the dashboard takes longer to go up. I mean, before, I would turn on the car, and in a short while, the needle would leave the Cold position, and after a while driving, it would stay slightly below the center point between Cold and Hot.

Now, when I turn on the car, it takes quite longer for the needle to leave the "C" indicator, and I can drive as long as I want, the needle always stays way below the centerpoint between Cold and Hot points, like a quarter of the way down. Can this be due to the oil that's is too thick or too thin?

Would you please share some light about why the Hot/Cold indicator is behaving different, and what oil should I use?

Happy New Year 2009!

Rick

ANSWER: I think there is a problem with the thermostat, try replacing the thermostat, this is not caused by the oil used. By the way, use 10w-40, the mileage doesn't matter.

---------- FOLLOW-UP ----------

QUESTION: Hello Ted,

Thank you for the heads up about the thermostat.
I don't understand why one oil or the other should be used... for example, if an oil thinner than 40 is used, what would happen, and if a thicker that 40 oil is used, how would it affect the car?

Thank you in advance,

Rick

Answer
Oil viscosity changes with temperature, the hotter it gets the thinner it gets and vice versa the colder it gets the thicker it gets so ambient temperatures are always a factor, in cold climates a thinner oil is needed, in hot climates a thicker oil. This is so because the engines rotating parts need oil as fast as possible, a 10w oil is thin and would provide lubrication sooner in cold weather but once the engine is hot this oil may be too thin to maintain the oil pressure the engine needs to lubricate all the parts, before multi-viscosity oil came along a either 30w 0r 40w oil was used depending on climate, this worked ok until higher performance engines came along, these engines demanded better lubricating qualities and this is why today we have multi-grade oils, they maintain the same lubricating qualities under different temperatures.
To answer your last question, if a thicker than 40 w oil is used it would limit the oil's ability to lubricate because the oil would be thicker when the engine is cold which would require more pressure to force the oil into all the enginge's clearances between bearings.