  We measured the temperature dependence of the probability of small heating
and total losses of UCNs on the PFPE Fomblin Y surface with various molecular
masses Mw=2800, 3300, 6500 amu in the temperature range of 100-300 K. The
probability of small heating sharply decreases with increasing Mw and
decreasing temperature. The probability of total loss weakly decreases with
decreasing temperature and takes the minimum value at Mw=3300 amu. As this oil
provides a homogeneous surface with minimal probabilities of small heating and
total losses of UCNs, it is the preferred candidate for experiments on
measuring the neutron lifetime.
