to answer your specific question, sometimes connecting a non-50 ohm load will impact the phase noise, sometimes not. If your concerned about it you could connect to the main oscillator output thru:
1) 3 db pad, to guarantee a good load
2) connect oscillator to a buffer amplifier. Unless you are measuring a very low noise oscillator, the added noise of the buffer amplifier is probably not going to factor in.
You do not want a varying load! Lets say there is a long coaxial cable between your oscillator and the measuring system, and that cable is vibrating (fan noise, etc), it could be a big factor.