Which test lead for HP 53131a frequency counter?
Even a 10:1 probe has a significant capacitance, probably in the 10 to 15pF range. 

In short, no probe will do if you plonk it onto a sensitive part of your oscillator, and expect it to read the correct frequency.

Any probe will do if you connect it to a buffered output, or a very insensitive part of your oscillator.

My favourite probe is a 470ohm resistor in series with a small capacitor, connected directly to a piece of 50ohm coax that plugs into the 50 ohm input. This way you have an AC coupled, more or less resistive, 20dB attenuation down to your counter, the price is right too. It presents a load of more or less 520 ohms to the circuit under test, which applied to a 50 ohm source, barely loads it. It's also good for 'scope inputs and spectrum analysers.