You have mechanical issue!Please excuse any ignorance on my part, I'm just trying to learn. According to Castrol's website, Castrol 0w-40 Viscosity cST @ 40 degree C is 66 and @ 100c is 12.7. Castrol 5w-40's is 69 and 12.8. Mobil 1 5w-40 cST @ 40 degree is 80 and 13.7 (Mobil 1 website.) Now I found this searching on Google "People also ask"...."Is higher viscosity better?"
The slope of the lubricant with a high VI is more horizontal: the viscosity remains more stable across a wider temperature range. This means a lubricant with a higher viscosity index is more desirable, because it provides a more stable lubricating film over a wider temperature range. Apr 17, 2020
This is in regards to the low pressure light blinking on my 2004 325i @ idle w/ engine warmed up. (Slight increase in revs, light no longer blinks.) This is the question I asked the BMW shop mechanic as to the cause and could it be oil related. (This is when he stated he would not use an "0" weight oil.)
On the next OC (in December), I plan on using the Mobil 1 Euro 5w-40 oil. Will be interesting to see if the oil pressure light continues to blink w/this oil? Will follow w/this info and thank-you once again for this wonderful website and all its member's knowledge and insights. Thank-you.