change oil...

[WeaponX]

your milage probably went down because of the change (if at all) of the viscosity of the oil, and not the kind (mineral, semi or full syn). thicker oils will tend to put a little bit more strain on the engine causing it to be worse with the milage.

learn how to pick the correct viscosity for your car... to thick or too thin is bad for the engine.

What brand/viscosity would best suit a 99 Civic Vti? I'm after increase in mileage too. Also, a change interval of 5K Kms is good for me but I would prefer trying 10K as long as it benefits me and my engine.

I'm not really familiar with viscosity so I don't know how thick or thin I should put in my engine. Does it depend solely on the engine type? Shell synthetic oil or the Casa Oil (can't remember the viscosity) is usually being used on my scheduled maintenance. I tried the 10K Kms oil of Casa but I feel that their regular or ordinary oil perform much better.

[orly_andico]

according to an article i read (linked from here!) all engine oils have a design viscosity of 10 at operating temperature. So it's not true that a 20W50 is "thicker" than a 10W30. They are both 10 at operating temperature. The 20W50 is "thicker" than the 10W30 at LOW temperatures. Which only happens when you're cranking the car. Even in subzero temperatures the engine will be at 100* Celsius or higher (of course! due to combustion..)