please smack that shopowner, you need a silicone oil not a petroleum oil. The shearing properties of the silicone oil is what causes the clutch to drag into action, the stp additive will be destroyed and will sieze up the fan eventually. A siezed up clutch fan is murder to a car, its not meant to work beyond 4000 engine rpm (you can hear it slow down beyond 4000 engine rpm), if it does overspeed you will see that it perfectly explodes and slices your radiator, hoses and bonnet too. (older BMW owners face that a lot)
silicone oil used in RC cars is perfectly usable too, Just pay attention to the cst rating of the oil. Toyota sell the silicone oil as a replacement part but charge a lot, the RC car oil is 4 times cheaper.