Very High Viscosity, but remember ... you may not be helping your motor that way. As long as there is an oil film on mating parts whether those parts should be under pressure or not, the oil is doing it's job. Higher viscosity oils may not be able to flow everywhere they're needed, when they are needed.
Personally, I have never been bothered by low oil pressure, as long as I knew the sump was full of oil and I had a reasonable reading. As engines wear, tolerances increase, and that is normal. Wide tolerance motors generally start easier than tight tolerance motors. In cold weather, they start much easier. This means less wear and tear on things like starters and batteries.
By "reasonable reading, as long as I have at least 10 PSI at idle, I am good to go. There is no real reason to need more, "just because".
Bends in oil lines or oil filter / cooler fittings (especially sharp bends of 90degrees or more) and various machined passages will raise oil pressure without benefitting the lubrication of the motor, in fact they are a detriment to it.
It's OK if you know the engine and it's tendencies and at that point find your oil pressure is unusually low or high, but if you are just panicking for no reason it's time to take a step back and find out what numbers you should be getting. Where you place your gauge or sender can affect the reading as well; if the numbers seem out of range, take the necessary steps to rule out unintended consequences that may be giving you an improper idea of what the pressure is at the critical areas (typically main bearings).
It's not difficult to drop the pan on many motors and have a look, do a measure, and even replace worn bearings before buttoning it up again.