creeze

I'm hoping somebody can give me some direction on a problem with a 1990 Cherokee with the 4.0. Getting constant smell of burnt oil coming through vents. Truck has 124,000 miles and burns very little oil. The valve and head gaskets show signs of seepage but nothing major. Just replaced entire exhaust save for manifold-there were no signs of oil burning on exhaust. Manifold appears to have absolutely no signs of oil on it-dry as a bone. The only thought I have is the valve cover could be leaking on the backside, ie, I should replace gasket-any advice or similar experiences? Thanks all in advance!