So I'm a having a few problems that may or may not be related. After being warmed up, my engine temp stays one or two marks below half (I forget the actual temperature). It never overheats, just always runs a little cool. Is only one or two marks a big deal? What could cause this?

My oil pan has always been covered in some oil, but I never had oil dripping on the ground, and I wasn't loosing oil, so I thought nothing of it. Recently though, there have been spots of oil on the driveway. The oil pan having oil already on it makes it hard to tell where it's leaking from. Is it likely just the oil pan gasket? Or are these trucks known for having a specific oil leak? (I know about coolant and the famous LIM). Any suggestions how I could pinpoint where this leak's coming from? I could just spray the engine with degreaser then hose it off, but it's -15 right now so I'd rather not do that...

My truck also leaks antifreeze, but I know it's not the manifold gasket. If I fill the reservoir to the line, it leaks, but only to a certain point. The engine never overheats and my heat works fine, also the coolant in the rad never gets low. Is it a small crack in the reservoir then? I tried to find a crack, but if it's just a tiny one then I probably would never be able to see it.

If I take the reservoir off, seal the necks for the hoses, then put Stop leak in it, would that be fine? I don't mean run stopleak through the whole system, JUST the overflow can.

Forgot to mention for my engine running cool, it could just be the thermostat right? Is there a way to check this?