How important is it that you add oil EXACTLY the same weight as the oil already in your car?

[raccoon]

Reason I ask, I have to add a pint or so of oil to my car about once a month. (other than that, it runs fine.)

I always try to add 5W 30 oil, but if it's a different weight, is it really a big deal?

[PoliticAverse]

1. No it isn't that big a deal. Try not to mix different brands of oil...

though, to minimize the possibility of getting incompatible additives.

[rurallib]

2. basically it thickens or lightens the viscosity

depending on whether the new oil is heavier or lighter