Engine Need advice

Clumps of brown substance under radiator cap. I was thinking it is oil mixing with water which is what I filled up the cooling system with as im having a continuing issue with lower intake gasket not sealing. My vacuum readings are fluctuating from 11 to 14 inches. Engine shaking and stalling at idle. I just replaced rings and bearings. Am running a tfs stage one cam with pro comp heads. Compression test. shows150 psi in all cylinders. The only code is a 136 which is left side o2 sensor showing a lean condition.

Yes I just got done doing that coolant system pressure test it held 15 for at least 5 minutes. As far as somebody else putting something in the in the engine it couldn't happen that engine was completely overhauled by myself over the last couple of months and I know there's been nothing put in that car. The only other thing I can think of is possible residual from the hot tank at the Machine Shop or maybe it's just rust because I've only been putting water in for coolant as I'm having a heck of a time with lower intake manifold gaskets and I didn't want the chance of glycol getting on the bearings. Correct me if I'm wrong but if the coolant ports on the lower intake are leaking wouldnt it result in coolant in the oil and not oil getting into the coolant system? I guess I'm going to have to put antifreeze in there to see if I get the white smoke out of the exhaust because with water it's hard to really tell if it's getting into the combustion Chambers so at least I could rule out that water is getting into the combustion Chambers because the engine is running rough as stated I only have one code for an oxygen sensor but I don't think that would cause it to run rough upon initial starting before it goes into closed loop

Got 150 psi in all cylinders for compression 5 speed trans. My last attempt was with a steel core gasket.