Coolant from Radiator going to Reservoir, but not returning to radiator on cool down.

[squash]

What make you so sure you don't have coolant in your oil. Have you had it tested?

[krogh]

Radiator pressure testing can sometimes lead you astray because most pressure testers do not test the sealing on the top of the filler neck or the small tube soldered into the neck for the overflow tube. The best way to pressure test is to use a new radiator cap and remove the lower seal that seals in the bottom of the filler neck, connect a pressure tester to the end of the overflow tube. You can use a push bike pump and old push bike tube valve stem to connect up to pressure the system to about 13psi. This type of test will test the full system including the overflow hose.

[krogh]

If you have checked for leaks in the overflow hose and replaced the radiator cap and it is still pushing water into the recovery bottle it is likely to be a head gasket or cracked head. To test you can use the radiator cap that was modified for the pressure test. You should also disconnect the fan belt that drives the water pump. Then put the overflow hose into a clear bottle to see if you have bubbles. Rev the engine and watch, if you have constant bubbles then you have combustion gas leaking into the cooling system. A very slight leak might not show unless you go for a test drive, make sure that the fan belt is refitted and you can use a long hose and put the bottle in the cab, but be carefull because it may get hot and burn you if the hose comes out of the bottle. The bubbles may show up only under heavy acceleration if it is a slight leak.

[Robo]

I like the idea of faulty rad cap.

Is the radiator gen or aftermarket, maybe something isn't to spec in size and not sealing correctly.

You noted in post 2 no oil or exhaust in water, so I assume if this is ok then,going out on a limb.
why dont you have the entire system vacume tested cold and see if ya can find a leak.
this will require a adaptor for a vac pump to connect to coolant system.

You have something closed when hot, but open cold.
so a vac test might give a result.
If the system cant hold a vacume, then you have a leak for sure.
A spray bottle or hose squirted may assist in finding said leak, listing for sucking noise with water should assist in detection.

[nissannewby]

Yeah as robo said could be an air leak as air is easier to suck than water so check all your hose clamps etc as they may not leak fluid but have a slight vacuum/air leak mine is doing the same so have to go down this same path of diagnosis.

[mudski]

What make you so sure you don't have coolant in your oil. Have you had it tested?

Shouldn't need it tested. Just drain the oil and you will see it. The two don't mix...
You obviously run the car without the cap on to see if any bubbles appear when it gets hot...I would change the hose to the reservoir and make sure all the clamps are in good nick and tight.
If it were me, I would change the said hose, tighten all clamps, and get the entire system pressure tested to see if it hold pressure. Atleast then you would have some sort of starting point.