have used previously. The high viscosity of Indopol H-8 ensured that thick fluid EHL films (h > 50 nm) were formed under all of the conditions used here. The fluid film thickness of Indopol H-8 was measured in an EHD2 rig from PCS Instruments (Acton, UK) using ultra-thin film interferometry, as shown in Figure . This technique requires the disk to be optically transparent, so a steel/glass contact, rather than steel/steel as used in the tribometer experiments, was employed. The measured fluid film thickness values were corrected using the equation due to Chittenden et al. to account for the difference in reduced elastic modulus between glass and steel, as well as the large loads (W = 600-1500 N) applied in the subsequent tribofilm growth experiments. The severity of the asperity contact can be quantified by the λ ratio, which is the ratio of the EHL film thickness to the composite RMS surface roughness, Rq. The Rq of the steel/steel ball-on-disk combination was 10.4 nm, so the λ ratios in this study varied between approximately 5 (120 °C) and 21 (60 °C). Since λ ≥ 5, full-film EHL conditions with negligible direct asperity contact can therefore be assumed in all the tribofilm growth experiments. This ensures that the stress conditions are well controlled and mitigates tribofilm wear and competing tribochemical phenomena. Unlike EHL film thickness, EHL friction is determined by viscosity in the central region of the contact, where the pressure is very high (> 1 GPa). Whereas lubricant base oils designed to give low friction such as PAO have low pressure-viscosity coefficients (α), polymers (e.g. poly(isobutene)) and traction fluids (e.g. DM2H) have high α values. Although the α value has not been reported for Indopol H-8, it is expected to be similar to the value measured for highmolecular weight poly(butenes) (α ≈ 30 GPa -1 ). Therefore, EHL friction (and thus shear stress) is likely to be sufficiently high to promote tribofilm growth on the steel surfaces in the accessible temperature and load range. To quantify the shear stress conditions under which the tribofilms were grown, we measured this quantity in ball-on-disk tribometer experiments using the Extreme-pressure Traction Machine (ETM) from PCS Instruments (Acton, UK). Figure shows that, in the absence of ZDDP, the friction coefficient of Indopol H-8 slightly increases with increasing load and decreases with increasing temperature. The EHL friction was also measured during the tribofilm growth experiments to ensure that this did not change due to the presence of the tribofilm. To convert the measured EHL friction coefficients to maximum shear stresses, they were multiplied by the maximum Hertz pressure (Pmax = 2.5-3.5 GPa) at the corresponding load. This approach provides the stress conditions in the central region of the ball-on-disk contact, where the tribofilm is formed.