Sludge in Coolent Recovery Tank

I have a 99 SL1 SOHC that has 125k. After a 300 mile highway trip yesterday there is a peanut butter colored milk shake consistency stuff in the coolent recovery reservoir. Sucked a little out and it smells like engine oil. Checked the oil and it's down about 1/2 quart, (full before the trip) but cannot see signs of water/antifreeze in the oil. I've never see head gaskets go and get oil in the antifreeze, usually the other way around antifreeze in the oil. Any ideas exactly what this could be?

Thanks

Cracked head. Remove the valve cover and inspect near each cam journal for hairline cracks. Most times it occurs between the 3rd and 4th cylinder.