Yeah, must admit the oil issue was driving me nuts with my D600. A lot of internet talk at the time was saying how it would go away after a couple of thousand shots but it actually seemed to get worse on mine the more time went on. Nikon ended up replacing the shutter twice and it was exactly the same afterwards.
It's a shame as I think in every other way, the D600 is a really nice camera and the sensor is a beauty.
Regarding the hand holding thing. You aren't going to get worse hand held shots than your D600. It's just that at 100%, you might see slightly more camera shake. At normal print and viewing sizes it shouldn't be relevant, you just might not be making the very most of the D800 sensor in those circumstances. When you can get the D800 on a decent tripod or have the light for really ....You can really see the difference between my 24-70 Nikon (a great lens I would say) and Sigma Art primes. You get much crisper detail out of the Sigmas. I guess that's true of all sensors but just seems to be even more apparent on these super hi-res sensors.