Distilled water and four silicone oils (Polsil OM 10, OM 50, OM
300 and OM 3000 produced by Silikony Polskie, Poland) were used as the lubricants. Polsil OM fluids are linear, non-reactive and unmodified polydimethylsiloxanes. They differ in their degree of polymerization and consequently in viscosity. The dynamic viscosity η at the test temperature of 25 1C was measured using the
Brookfield HADV-III Ultra viscometer with cone/plate configura- tion and is provided in Table 1.