What should my oil pressure be?

10psi for every 1000 rpm is the rule of thumb. Make sure you get a pressure gauge T'd next to the sender. With that thin of oil, your readings are high. The pressure bypass may need attention. Also, check on the ZDDP content of the Castrol. Why the 5-30 weight? Common would be 15-40 or 20-50

Question does the 93 Pro Boss have a roller camshaft or a flat tappet camshaft. If it is a roller camshaft then it will not matter how much ZDDP the oil has in it. I run the 15-40 Amsoil is our boats it is good for either flat tappet camshaft or roller tappet camshaft.

70 is too high.
As advised, measure with a hard gauge plumbed into the oil pressure sender port.
May have faulty sender.
Also, unless the motor has been rebuilt with super-tight tolerances (like modern engines), then use 20w-50 non-synthetic oil.

The 10psi per 1,000 RPM rule goes back many years, I believe Smokey Yanuck coined that term, and it does take care of an engines needs but newer engines are built better and even oil pumps in 1970 had enough volume by 400 RPM to already start opening the by pass.
I like to have an engine tight enough to hold steady 40psi at idle and max at 60 psi wide open. If the tolerances inside are good this works well.
When I build one with .002 clearance on the Rods and Mains it generally holds the oil pressure right at the By-Pass rating cold and hot with a stock oil pump, not a High Volume pump.
Marine engines are generally a little loose compared to automotive so the idle oil pressure will be slightly lower due to more spill.
10 psi at idle will work but it also tells me something is very loose in the engine. Cam, Main or Rod Bearings.
It would help to know the history of this engine, did it always have high Oil Pressure or did someone rebuild it and install a high pressure oil pump.
With that high Oil pressure I would not go to a thicker oil but as mentioned make sure your oil has the additives necessary to protect your camshaft.
You can buy Diesel Oil at Walmart for $20 a gallon that has all the additives you need.
They have added a 10-30 Diesel oil in Delo Brand (Chevron) which used to only be available in 15-40W, this is good oil and would work. I use it because it has what the engine needs and is way less than the $8. per quart for the fancy oils.
If you switch to 20-50 you might have 100 PSI in this engine I would not do that.
I have seen engines with high oil pressure twist the oil pump drive shaft into spaghetti on a cold start and I saw one blow the spin on oil filter housing right off the engine.
Modern race engines are running thin oils, not the thick oils like were used in the 50's, 60's and 70's.   I know racers using "0" weight in race engines. They have found as much as 30 horsepower advantage using light weight oil.   These engines were clearanced for these light oils. You cant' just decide to start running the super light oils. My only point is the comments on needing 50 weight are old school and 20 years past current thoughts.
You do need ZDDP in these flat tappet engines as advised but you don't need heavy oil.
My 1978 Nautique 351W gave me 1,500 hours great service on 10-30 weight 30 years ago. It needed new heads at 1,100 hours but the lower end was still rocking when we sold it. Clean oil with ZDDP is important for survival.   

Steve,
Oil viscosity selection is really how the engine is built and how "tight" it is as mentioned above. Temperature doesn't come to play since marine engines these days have thermostats. (old marines did not!!) Also regarding temperature, keep in mind unlike a car, we have an unlimited supply of cooling available with the lakes we run. I do suggest getting some oil with some decent ZDDP levels to protect the flat tappet engine. I run 20-50 VR1 Valvolene in all my boat engines but there are other good oils as mentioned.

I wouldn't get too fizzed up about high-zinc oil. Yes, it's better for older motors with flat tappets, but yours has hydraulic flat tappets, not solid, mechanical ones.
The hydraulic lifters to provide a cushioning effect to minimize lifter-to-cam abuse.

Is 20-50 really necessary or better in a pretty stock flat tappet engine? VR1 is made in 10-30 as well and would seem like a good choice in places where the temps rarely exceed 90.

I don't know if it's necessary, but I've run VR1 20W50 in my '63 V6 Buick, '98 GT-40, and '86 454 and none of them shows excessive oil pressure. In the '63 it helped keep pressure up at low power levels. I run them in pretty cold water/air at times and still no issues with excessive pressure. I really expect Steve to find out his gauge is off.

Steve, what oil filter did you use?

Hate to say it but the Fram filters are some of the worst on the market.   I would try a wix or a purolator pure on oil filter.

I finally got to check this stuff out.

The minute I pulled my dash out to get to the gauges the voltage gauge went to 12 so I'm thinking the ground was a little jacked up.

The oil pressure remained at 80 indicated and when I jumped the ground it did drop to about 70 indicated.

I used the external gauge to test it and it indicated 40-60 between idle and about 3500 rpm - so I'm thinking I'm fine on oil pressure? perhaps it's the indicator or the gauge?

Now back to the oil debate, it seems that ZDDP is needed - what weight would you recommend now in the high ambient temperatures where I'm running?

and oil filter now that 1 says Fram stinks?

I know a lot of people here do not like synthetics. But I like Amsoil they make a real good 15-40 Marine oil . One added plus with this oil it is a mil spec rust preventative for when you have the boat just laid up. It is also a diesel oil. They also make a real good oil filter the one you would need is a EAO-15. I have been running these oils for years with awesome results. Hope this helps. Some people think that it is a waste of money, but I have many motors with high hours that run strong as new.

first boat wrote: I'm thinking the ground was a little jacked up.

SNobsessed wrote: I'm not following adding a ground to the dash gauge. I thought the ground was at the engine sensor.

Correct. The ground source is the engine block. The only reason there's a ground on the gauge is for illumination.