Here's my 2 cents,
Always use multigrade oils (like 10w 30, 15w 40 etc) reason:
Oil is thick when cold and thin when hot. When you start the engine it s cold (~25 - 30 C here) and the oil is thick. So it s hard for the oil to go through the engine and lubricate properly. So we need a thinner oil when starting up. When the engine heats up oil gets thinner. So if we use a very thin oil it's better for start up but will be like water when the engine is at the operating temperature(~100 C). A thicker oil (like 40 grade) is good when the engine is hot but increase the engine wear during startup.
This is where multigrade oils come in. They are thin enough when cold to lubricate during cold starts and thick enough at the operating temperature to properly lubricate.
For what I've read, even the 0w-XX oils are not thin enough for start up in the morning so lower the W number it's better. Also 90% of the engine wear occurs in the time that takes the engine oil to reach the operating temp. That's why they tell you to not to rev the engine immediately after starting.
If you use a better oil it'll hold up it's viscosity better in hard conditions without breaking down will clean up the engine properly.
Always Use a high quality multigrade oil and change regularly and it s the best thing that you can do to ensure the longevity of the engine. Synthetics are even better if you can afford
Another thing not all 40 oils have the same thickness. The 40/30 actually defines a thickness range. So it is possible for a one 40 grade oil to be thinner/thicker than another. (Get the oil spec from the manufact's site if you need).
I currently run toyota 10w 30 on my 96 corolla CE110 (diesel). It's a good oil and gives good acceleration and fuel economy. Last time I checked over 500km of driving , the car had done 15.75 km per litre. (mixed driving city & out).
But it consumes a bit of oil. (has got 190,000+ on the odo). So I'm thinking on moving to mobil delvac 15w 40 with the next filter change. I guess 30 is too thin as the engine is a bit worn out.
I change oil every 5000km and filter every 10000km.
So the summary : Always use a good multigrade (synthitic if possible) oil and change regularly.
