I just did my oil change today.
I wanted to switch to Castrol for this change as it is a premium oil, but couldn't find the Castrol GTX 10W30 synthetic blend (semi synthetic).
So, I disregarded all advise and went for Castrol Magnetic 10W40, I think it is full synthetic... (sorry)
My Jeep is a 2006 inline 6 (4.0 liter), and I have almost 220K miles on it (350K kms).
The average ambient temperature in the country i live in is
15 - 20 degrees Celsius in coldest winter
45 - 58 degrees Celsius in hottest summer and maybe more
I bought my jeep with less than 15K miles used different oils since then conventional, synthetic and semi synthetic.
My previous oil that i was running was 5W30 Synthetic blend.
My EOP was:
45 psi on cold start
32-34 psi on Idle (engine hot)
54 psi on 2300 RPM (approx 70 - 75 MperH)
Max Psi is 59 @ full race which i guess the capacity of the oil pump.
Today after changing the Oil to Castrol 10W40 magnetic (Full Synthetic), the readings are almost the same, but i still have to check the readings at cold start.
I am not a mechanic, and most of my knowledge comes from the internet and forums like this, but i have done a lot of work on my Jeep through a professional work shop owned by a close friend, and I enjoy watching upclose and asking him many questions