slow coolant loss

It's a 93 with a 4.3L 6 and 176,000. It's losing coolant slowly. I've added coolant every few weeks and I drive the truck about 400 miles per week. There's nothing on the driveway, and nothing in the oil. The coolant is green like it should be. I have noticed some green slop on the intake manifold. Could this be due to bad intake gaskets? I've sprayed the intake down with carb cleaner to test for leaks and haven't found any. The truck is running well and all the gauges read normal.

Pull all the spark plugs and check each one for coolant contamination. Since you have coolant evidence on the intake, the intake gaskets are most commonly the cause.