Hi Steve

The coefficient of thermal expansion is what is used to compare the concept you have described here. I do not have info on the raw material we use but as a proxy I have found some data on line. For PAO's (https://www.tri-iso.com/documents/exxon_mobil_spectrasyn_2c_TDS.pdf ) lets call it 3.6 x10^-4g/ml.°F . This converts to 6.5 x10^-4g/ml.°C . The coefficient of thermal expansion for mineral oils is 6.4 x10^-4g/ml.°C (http://www.pantherlubes.com/tl_6.html#:~:text=The%20values%20of%20the%20coefficient,%2D4%20%C2%B0C%2D1.) and 7.0 x10^-4g/ml.°C for Oil (unused engine oil) also 7.6x10^-4g/ml.°C for Paraffin oil (https://www.engineeringtoolbox.com/cubical-expansion-coefficients-d_1262.html). 

So if we round off and average we are comparing 6.5 (PAO) to 7.0 (mineral oil) (x10^-4g/ml.°C). This means that mineral base oils expand about 7% more than PAO synthetic base oils((7.0-6.5)/ 7.0 x 100). However if we assume 1/3 of a formulated engine oil is composed of additives and the same additives are used in mineral and synthetic engine oils then 2/3 of 7% is 4-5 %. This means that mineral engine expand at about 4-5% more the PAO synthetics over the same temperature range. 

Of course this is very approximate and obviously dependent of the values found in a quick look online. You will have to judge it's significance on any application you are examining.

Kinematic viscosity is a measure of a fluid's internal resistance to flow under gravitational forces and this more complex than thermal expansion and is fundamentally affected by the types molecules in the fluid. You can consider Viscosity Index (VI) in this context ( http://www.penriteoil.com.au/knowledge-centre/Oil-Composition-and-Additives/44/Viscosity-Index-Improvers-(VII)/178 ). The higher the VI the less an oil thins as it warms and the better it can provide hydrodynamic lubrication. 

To my mind the way to over come any issues is to follow the correct procedure using the correct oil and issues that are raised here resolve themselves.

Cheers

Costa

Penrite