Rayzor said:
Will the lack of use also cause the discolouration?
Lack of use will not cause discolouration.
Lack of oil changes will though.
Typically the car will be started allowed to idle and get hot and then switched off.
This done repeatedly without the concomitant oil changes will present with fuel contaminated,old oil and the discolouration soon follows.
Equally when a car is driven very hard and then switched off immediately,it will allow the fumes to settle on the components to the degree where one cannot move a cam follower on the shaft to change shims. (Had one of those).
Best practice would be for oil to be changed annually on cars that do not see use but are started regularly and left to idle.