w126 420SEL oil pressure question

[davestlouis]

I did the first oil change on this car since I bought it, and I'm nervous about something...
the PO used 10w40 Havoline and Fram filter at an unknown change interval. The pressure gauge would read "3" at speed and under load, and fall to right around 2 at idle or coasting

I filled it with 15w40 Delo and a Wix filter today, in 60 degree weather. The pressure gauge pegged at 3 and stayed that way until I got the car up to temp, then it only fell to 2.5 at idle or coasting.

I did note that the Delo seems noticeably more viscous than the 10w40 Castrol GTX that my wife's Jeep uses. Is it just an oil difference, or have I screwed something up?

BTW, the car has 172xxx miles.