28 psi is nearly 2 bar, I would be fine with 28psi at hot idle. Did they mention if the pressure readings were at idle or 2k rpm?
Do you know which oil they put in? It should be either 0W40 or 5W40. A thinner oil can cause lower pressure. Depending on your location, (like us down south), you can run even thicker oil if you don't have cold winters.
Investing in an in-car pressure gauge would be wise at this point.
Here are specs I found online from a great writeup:
Factory Oil Pressure Specifications:
Oil (not coolant) must be at normal operating temperature 80°C
Pressure at idling speed should be a minimum of 1.0 bar (14.5psi)
Pressure at 3000 RPM should be between 3.5 to 4.5 bar (50 to 65psi)
And here is the link. More info than I have ever seen on the subject:
1.8t Oil Pressure Survival Guide Oil Pressure Warning Light diagnosis and troubleshooting Table of contents Introduction Section 1 - Main Oiling System Components of the 1.8t engine Section 2 - Understand Oil Flow through the Engine Section 3 - Basic Physics of Flow and Pressure...