<stevenqrdh@gmail.com> wrote
> 2001 Civic LX - Supposed to use 5W-20 (I think that's what
> it says on
> the cap). I used 10W-40. It's what a mechanic recommended.

What weight does your owner's manual say to use?

I would bet it specifies 5W-30.

Using an oil that is too heavy reduces fuel mileage.

Too light, and engine wear may accelerate.

> Not the 1st time I've used a diff oil than spec. Just
> wondering if
> there can be bad consequences if I use the wrong motor
> oil?
>
> Also, what exactly do those numbers and letters stand for?
> I think it
> refers to viscosity but I don't remember which number?

Googling will turn up much discussion of this. Try, for one:
http://en.wikipedia.org/wiki/Motor_oil

Synthetic oil is a whole other discussion, but probably a
relevant one for you.