Brown-ish Antifreeze

[WestButteTruck]

My coolant in the reservoir looks real brown, almost oily. Could be sludge of some sort I suppose. I really have no idea if it's ever been flushed or not. But was wondering if it could be oil and how to tell. I changed oil last fall before I put it away and have probably put less than 500 miles on it since oil change. But, it was down over a quart and nothing on the floor. Could the intake gasket be bad? Just changed trans fluid, nothing extra there. The fluid in the overflow was well above the "cold" mark after sitting all weekend.

Pickup is an 85 with the 305 and 69,xxx miles.