5.0 oil viscosity

i used 10w40 castrol

I put 10w-30 amsoil in my 5.0 engine. I use 10w-30 oil because it has the least amount of polymer's. The wider the weight number (10w-40)(20w-50) the more polymers the oil has in it and polymers have no lubricating properties. polymers are man made molecular chains that make the oil act like a thick oil when hot or thin oil when cold. Just put 10w-30 oil in and be done with it. I used to run straight 30 weight oil in my chevelle, zero polymers.

While i've always used 10w30.
You can't go by the factory oil pressure gauge, it's completely useless. I have the stock sender car hooked to my 03 cobra engine that makes 80-90psi on cold start up (settles at 20-25 hot), doesn't peg my stock gauge.
Get an autometer full sweep mechanical gauge.

This is how oil works.

Oil is designed to reach optimum lubricating properties at regular engine operating temperature. In other words, you select an oil for how it acts when it's "hot". NO OIL ON THE SHELF works at "optimum" when cold. When the oil is "cold", it is too thick to achieve optimum lubrication. This is why they say that the most engine wear occurs right at/after startup, because that is when the oil is "cold".

The first number represents what weight the oil "acts" like when cold. THE LOWER THE NUMBER THE BETTER. Remember how I said that the oil does not work well when cold? It is too thick when it is cold, so we want it to act as "thin" as possible when it is cold.

The second number represents what the oil acts like when hot. This is the number that actually would effect hot running oil pressure, NOT the first number.

So in short, 5w-30 IS ALWAYS the better option over 10w-30. 5w-30 is better for your engine at startup, yet acts exactly the same once the engine is warm. Your engine doesn't know the difference between 5w-30 and 10w-30 once it's warm.

So, if you are experiencing low oil pressure when the engine is hot, you need to switch up to a Xw-40, because that will actually have an effect.

Here is a good article for reference:

Ferrari Owners Club Florida Region - Articles

Good posts above.

Low oil pressure is a sign of bearing wear. The clearances have opened up so the oil kind of just flows out more throughout the engine. There are advantages and disadvantages of thin and thick oil. To make it basic, thick oil protects better, thin oil flows better. 10w30 was the weight the car was designed to run. Although a 30 weight was ideal when the engine was new, with some miles on it, the clearances have opened up a bit, so a 40 weight might be better, they aren't that far apart. I wouldn't run a 50 weight oil (20w50) unless you had a stronger oil pump driveshaft.

Another option would be to run a higher volume oil pump. Not the same as a high pressure oil pump. The lower pressures will mean that less oil is getting to some parts of the engine, mostly the top where it isn't needed that much. The end result is more engine wear. I would just run a good oil because there is no way around it, the engine is worn and is wearing even faster. Eventually it will need a rebuild. Until then, just make sure it has good clean oil and it should last you a while.

I was seeing low pressure on my 91's stock engine (15psi @ idle with a sport comp). So I switched from 10W-30 to 10W-40 and picked up 10 psi @ idle. However in your case, since you are still using the stock gauge, you are just shooting in the dark. Get a proper gauge and you will have a better idea of what is going on inside your engine.