Late 1999 4.8 Liter V8 GMC Sierra Seem to be losing small amount of coolant. No visible leaks, no power loss, engine is smooth and runs excellent. No water in oil, no oil in water, no overheat (runs around 190 degrees according to in dash gauge ). Heater will not get hot, barely warm. Replaced blend door actuator, no difference.

The first thing you need to check is for visible leaks, once you turn your vehicle on, there have to be some kind of leaks, pay attention since if the leak drop on top of the manifold, it will evaporate and you won't notice it.
Another thing to look for is excessive heat, with the vehicle on, put yourself in front of the vehicle, if you feel an excessive amount of heat, then probably the head gasket needs replacing, and it evaporate the coolant.
So as you can see, if there are no visible leaks, it is probably a problem from the inside of the engine.

if the coolant is coming out from in between the transmission & the engine it usually means that one of the freeze plugs on the back of the engine is rusted out. keeping the right amount of coolant in the cooling system keeps them in good condition longer. when mine went out I had them replaced with brass plugs, they will last longer.

5w30 in cold months 10w30 in warm to hot months. High milage if its over 75,000 miles.

You will need to figure out if it is fuel or ignition related.
If you have not done a tune-up by replacing plugs and other parts within the past couple of years, start there. Including the fuel filter.
Is the check engine light on ?

milky sump oil is coolant in the sump
could be head gasket
could be the aluminium housing behind the water pump has corroded and leaking water into the sump
that will account for you thinking that you need anew radiator

These motors are famous for the intake manifolds leaking coolant.Have a pressure test put on the coolant system and look for a leak around the intake manifold corners.Check all four corners i've fixed many of these.

Hello Admin
The location of the thermostat is on the right front side of the engine about half way down on the end of the radiator hose.
It is visible on the diagram below just above the AC compressor. The bolts that retain it are also shown.

That's not oil level you see on the gauge -- that is oil pressure in pounds per square inch (PSI). It is normal for the pressure to fluctuate when the engine RPM goes down. If it stays on 60 PSI on the road, you are very lucky -- that is excellent oil pressure!

Check the oil, and make sure it is not turning a milky brown color. If it is, then you probably have a leaking head gasket/cracked head/cracked engine block. I normally wouldn't recommend this, but put some stop leak in it since the leak is not severe. Hopefully it will take care of it.
Good Luck
Chris