1999 Ford Mustang Oil In Coolant

Leaking cylinder head gasket is rather unlikely. It would be more common, if there was a leak between the cooling passages and the oil passages, that 15 pounds of cooling system pressure would push coolant into an oil drain-back passage where there's no pressure. A better suspect, but only if there's a lot of oil in the cooling system, is a leaking transmission cooler. Some cars use external transmission coolers which would cause a fluid leak on the ground, but most use a coiled tube inside one radiator tank. The two fluids will mix both ways because at times the transmission cooler will be under pressure and at other times the radiator will under pressure. The clue is the transmission fluid will be brown. If that fluid is bright red, and you just have a little oil floating in the radiator, you likely don't have a problem of any significance.

Please forgive me for overlooking the manual transmission. We know we can rule out a leaking cooler inside the radiator. If you don't see oil floating on top of the coolant in the reservoir, it is likely you don't actually have a problem. It's a better bet you're seeing some residual additive in the antifreeze or perhaps a little grease or assembly lube from the water pump.

Regardless, if you don't know the history of the coolant, consider having the cooling system flushed and refilled. Antifreeze will always be antifreeze, but it's the additives in it that wear out in about two years. Also, it is normal for the coolant to become acidic over time and that leads to corroded radiators and heater cores. Antifreeze contains corrosion inhibitors and water pump lubricant. That's why we replace it every two years.