At the recent oil change found a light deposit of mayo-like looking substance on the shoulder of the oil-filling pipe.

 

This is a 99 Legacy Wagon, 2.2L, manual trans., 166K miles. When I bought it in Oct, 2003 with 127K mi, I startet using synthetic oil and still use it. There is oil seepage from I think the rear part of the head gaskets, that I intend to take care of this summer.

 

Any clues at what this substance means?

Does it look something like this;

 

 

 

 

If so then it's normal. When the engine cools, moisture condenses in it. When you fire it up again an emulsion forms and and collects in coller locations like at the mouth of the fill tube. Jeep has a TSB out about for the Grand Cherokee which uses a baffle to minimize the collection of this emulsion.

I agree that some build up is normal. However, only in vehicles that run very short distances and never get fully warmed up. Especially in cold climates. If this description DOESN'T fit your car, you may have a small head gasket leak.