Sure sounds like your problem is in the electrical gauge. And one other contributor to that situation would be that the alternator output voltage will drop as RPM drops low, and will also drop as the alternator gets hot. If the alternator is on the small side and not up to the job of supporting the current draw of the fan, then that will make it even worse.
And FWIW, when the oils get really hot, the synthetics will maintain as good a viscosity as a dino oil, and when reeeealy hot, like under hard endurance racing conditions, will maintain viscosity better. So a hot temp pressure issue is not gonna see change much if at all with an oil type change, just a hot viscosity change. At lower temps, then the dino oils will thicken up more as temp drops to what we typically consider low oil temps (like 100F), and so would show higher pressures then.
