Let's start with a parts breakdown for you:
How much oil is it using?
What color smoke? Does the oil in the sump smell like gas?
Dark smoke is unburned fuel. Oil will be bluish white. Does the oil in the sump smell like gas?
By more fluid in texture, do you mean the oil seems thinner? Does the oil level appear to be increased? What does the spark plug look like
From your description, I believe you have more than one problem here. The thinning of the oil is most likely that you have a bad needle and seat in the carburetor. When the unit sits, fuel will flow through the carburetor and then into the cylinder, past the rings and into the sump, mixing with the oil. This often will cause the oil to pumped up through the breather into the carburetor through the breather. Check the float by shaking it. If there's any fuel in the float, replace it as well as the needle and seat.
I also suspect a blown head gasket between the cylinder and the push rod tubes. Either that or worn rings. The dirtiness of the oil tells me you're getting exhaust blowby down into the sump. Head gasket is the most likely (and least expensive). I think you'll need to remove the head and take a look. Be sure to check the intake valve stem seal while you're at it.
Considering you've got a 15 year old engine, that's not bad. You must have taken really good care of it. Most don't make it half as long.
I hope this is helpful!