## 2 Materials and Methods
### Instrumentation  
The evolution of the permanent gasses, hydrogen and methane, was monitored with a MicroGC 4900 Varian, equipped with a molecular sieve (MS5) column for hydrogen detection and a poraplot Q column for methane and volatile hydrocarbons; in both cases a TCD-type detector was used, with Argon functioning as a carrier gas. The precision of the obtained data was conservatively evaluated according to an estimation of the instrumental errors provided by the GC calibration curve for hydrogen.  
Degradation products condensable at room temperature were analyzed by an Agilent Gas Chromatograph coupled with a mass spectrometer, and it has a capillary column (Alltech Capillary Column Phase: AT-WAX), with a length of 25 m, an external diameter of 0.53 mm and a film thickness of 1.2 \(\upmu\)m. A NIST mass spectra library [32] was used to identify the compounds characterized by the most intense peaks. Each sample (100 ppm) was prepared in cyclohexane and analyzed under 0.7 mL/min of Helium as carrier gas, imposing as the temperature program a ramp of 10 \({}^{\circ}\)C/min up to 500 \({}^{\circ}\)C.  
A thermogravimetric analysis was performed to evaluate the volatility of the oils as a function of temperature with an accuracy of \(\pm\)1 \({}^{\circ}\)C. With this purpose, the TGAoutput curves were analyzed considering the onset degradation temperature as the value at which the oil loses 3% of its initial weight. The measurements were carried out both under nitrogen and air atmosphere using a Mettler Toledo (Greifensee, Switzerland) (TGA/DSC 1) thermogravimetric analyzer. Around 20 \(\mu\)L of sample are placed in a 70 \(\mu\)L alumina crucible and positioned into the furnace.  
Specific heat was measured with a Differential Scanning Calorimetry (Mettler Toledo DSC1), according to the DIN51007 methodology [33], using a Sapphire standard. Measurements were carried out from room temperature up to 240 \({}^{\circ}\)C (ramp of 10 \({}^{\circ}\)C/min) in 40 \(\mu\)L alumina crucibles under nitrogen (20 Nml/min).  
Viscosity measurements up to 100 \({}^{\circ}\)C were performed with a rotational-type rheometer (TA AR2000 ex, TA Instruments, New Castle, DE, USA) with parallel plates. Viscosity was evaluated at four different temperatures, from 40 \({}^{\circ}\)C to 100 \({}^{\circ}\)C, over a programmed shear rate range (from 100 s\({}^{-1}\) up to 1000 s\({}^{-1}\) and vice versa).  
Both for specific heat and viscosity measurements, ten different measurements were carried out for each sample to evaluate the precision of the results (Equations (4)-(6)).  
\[X_{m}=\frac{\sum_{m=1}^{10}X_{i}}{N} \tag{4}\]  
\[Err_{m}=\frac{2*\sigma}{\sqrt{N}} \tag{5}\]  
\[Err_{\gamma_{w}}=Err_{m}/X_{m}*100 \tag{6}\]  
where  
\(X_{m}\) = average measured value [J/g*K]  
\(X_{i}\) = measured values [J/g*K]  
N = measurements number  
\(\sigma\) = standard deviation  
\(Err_{m}\) = mean error  
\(Err_{\gamma_{w}}\) = precision percentage error  
Instrumental accuracy, for viscosity measurement, was calculated in accordance with Equation (7) [34] using a standard material known as N100 [35].  
\[Err_{\gamma_{w}}=\ \frac{(|X_{Th}-X_{m}|)}{X_{Th}}*100 \tag{7}\]  
where  
\(X_{m}\) = average measured value [Pa*s]  
\(X_{Th}\) = value from technical data sheet [Pa*s]  
An infrared analysis was used to track the thermal oil behavior by overlapping the spectra obtained from fresh (unheated) and aged (heated) oil samples using a Perkin Elmer spectrometer with an operative wavenumber range of 350-7800 cm\({}^{-1}\). The device is equipped with an ATR (attenuated total reflection) module with a zinc selenide (ZrSe) crystal.