Oil pressure no longer is not really a major factor for engine component protection. It is still important but for a naturally aspirated engine which isn't overly stressed like the CG13 what is more important is a quality oil which will hold its own when under stress (there's heaps to it but i cant be arsed going into it)
5w40 is a bit thin, a 10w40 is more common and does the job fine.
I would only step up to a 50 weight oil if the motor is tired. I used this in my K11 when it had less than 140,000km on the clock.. It drove and sounded better with 10w40. The 15w50 made it feel really old and lethargic for lack of better adjectives.