You don't have to use Rotella. Any oil with a primary API service category of CJ-4 is fine. Chevron Delo, Mobil Delvac, and Shell Rotella are the major national brands, but there are many house brands including Walmart's SuperTech, which is usually cheaper.

I think your oil pressure it okay. My W201 owner's manual says as low 0.3 bar at idle is acceptable. My five-speed typically pulls 1.5 bar at 700 idle speed, but the lower idle speed of autos will reduce oil pressure. The lowest I've seen is about 1 Bar in hot weather, heavy traffic.

15W-40 CJ-4 is okay down to consistent cold starts as low as about 15 deg. F. Below that use the 5W-40 "synthetic" CJ-4, but it's more expensive.

The following link has everything you need to know about selecting engine oil for vintage engines with sliding surface valve trains. The article starts on page 3.

http://www.stlouisncrs.org/news_files/St_Louis_NCRS_Chapter_July_2011.pdf

Duke