voltage step down to 0.15v?

Use a 5K ohm 1w potentiometer as a voltage divider.

The diagram shows a potential divider that will always output over 0.15V if you use 1% tolerance resistors. It assumes the sensing device does not have a low input impedance.

Guaranteeing to get very close to 0.15V is not possible with normal resistors because 0.15V is only 0.25% of 60V and thus 1% tolerance resistors have a relatively big variation in possible outputs. Thus I have aimed slightly high such that the lowest voltage outcome is still more than 0.15V This should be fine if the sensor is only doing a binary test for more or less than 0.15V

Fan sensors are frequently open collector hall sensors, in effect a switch to ground.

If you try to measure the output with a meter, you will typically get a very small voltage reading. 

The "switch" pulses in time to the rotation of the fan rotor; it would have a resistor to the device logic supply voltage (eg. 5V) so the device can count the logic-level pulses to read the fan speed.

0.15mV is ludicrously low, I cannot believe anything is actually using that as a sense voltage.

If you wanted to recreate that to try it, use a 1 megohm resistor from 60V in series with a 2.7K resistor to ground.

The junction of the two should be at around 0.15mV