Hi.
20:50 oil is required in a Stag V8. The 15:40 you initially added is simply too thin. I suggest you drop the whole lot out, replace the filter and refill only with 20:50. The pressure will improve with the thicker oil.
Pressure at cold should be around 40 to 50psi and that will drop to 25 to 30psi when the engine is properly hot and the oil is nice and warm.
With regards to the company you bought it from, they sound unprofessional. If it was me I would be thinking what else did they miss on the service! But that's probably just me.
Enjoy your new acquisition
Regards
Bruce (Stag owner for 18 years)