Oil question

[j1b3h0]

Just got my '88 560 SEC back from the shop for an oil change and some other things. The shop asked, and I specified synthetic oil. When I asked what kind of oil they put in the car, the shop told me Castrol Euro Blend 5W-30. The owners manual specifies 5W-30wt only when it's 14 degrees F or below-unlike the SF bay area where the car lives. The car doesn't leak and has 171,000mi. Oil pressure seems normal-the gage is usually pegged but at idle when warm drops to just below 2bar. When I balked at the viscosity, the shop manager told me they put it in everything and MB recommends it even for my car. Do I risk permanent harm by leaving this oil in the car, or should I change it pronto?