Mineral oil loses viscosity when warm

[RCn]

A friend of mine is an experienced Harley rider / builder and also a very skilled mechanic (which is useful when riding certain brands). We discussed my ongoing project of mounting an oil pressure gauge on the Vmax.

On his latest build he mounted a gauge which is common on HDs. He noted that the drop in PSI was significant when runinng on idle and hot oil and using mineral 20W50. The Motul 5100 10w40 ester based oil which seems to be a semi synthetic had much lower drop.

Does anyone have an explanation of the phenomena or a similar experience?

 

[MaxMidnight]

Short answer is I don't know but...
You aren't comparing like with like as the viscosity of the oils are different and it may be as simple as that which explains the difference?
Your semi synthetic is less viscous when cold therefore it would be reasonable to assume it would flow easier when cold which would equal a lower initial pressure than the thicker mineral oil. This would level out when they are at operating temperature.
A more valid comparison would be using oils with the same viscosity rating.