Outer viewfinder smudged? How to safely clean?

Hey guys,

I recently bought a Nikon D90 (used from a dealer).

The camera is great but it does have some oily smudges on the outer part of the viewfinder (where the eyepiece goes). It looks like someones presses an oily finger or nose against it? It doesn't affect the image but it is annoying (I am a perfectionist).

Anyone know how to clean these safely without scratching the tiny viewfinder glass?

I was thinking microfibre cloth?

Cheers,

Rob