[Rick Norwood]

When you pull the 1-3-5 head off I think you'll find the culprit. When you get the gasket off the 1-3-5 head look very closely and compare it to the 2-4-6 gasket. I really doubt you will see a large gapping hole in the gasket. It will be a small burned area on the gasket between the water jacket and the cylinder (#5 I bet).

I know you said that there was no water/antifreeze in the oil, but here is an old trick for those who may ever need to "flush" their crankcase due to "chocolate milk" in the oil.

Completely drain your crankcase and remove the filter. Go down and buy a gallon of Diesel Fuel from your local Gas station (yes, right out of the pump). Install a brand new cheap filter and Fill the crankcase with the Diesel fuel taking care not to overfill. Remove all of your spark plugs and pull the distributor wire out of the distributor and crank the engine (you don't want any sparks) anyway, I cranked mine until the battery died. DO NOT UNDER ANY CIRCUMSTANCES START THE ENGINE WITH THE DIESEL FUEL IN THE CRANK CASE!!! Drain the Diesel fuel and remove the filter, leave the oil drain plug out for a while and pour a quart of oil into the oil fill hole and let that drain too. After that, close everything up, recharge your battery, re-install the plugs, and fill with fresh oil and another new filter, and start the engine. Run for a few minutes and check the oil again. I would only leave this oil in for a day or two, then change it again. If you fixed the source of the water leak, you should be good to go after this.