M104s are known for head gasket problems. No head problems AFAIK.

Putting too much oil in the engine will not cause the head to crack or otherwise fail.

If the engine oil level is fine than it's probably transmission fluid. Unfortunately you can't check the transmission fluid level since you can't start the engine or get the transmission to operating temperature. And you probably don't know what the level was before you found that gunk in the reservoir.

Sixto
... stay tuned