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 mass ̅ (2800, 3300, 6500 аmu) in the temperature range of 100-300 К. The probability of small heating sharply decreases with increasing ̅ and decreasing temperature. The probability of total loss weakly decreases with decreasing temperature and takes the minimum value at ̅ = 3300 аmu. 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.