Chevrolet Repair: 94 S-10 Blazer oil pressure drop at high temp., motoroil, engine speeds

Question
I just finished the longest oil change of my life.I drive a 94 T-10 Blazer with a 4.3l V6 4x4, The oil change place I last went to put the filter on with a ratchet which made it impossible to remove. The tools I had crunched the filter trying. I took it back to them an they removed it and changed the filter. I completed the oil change with 5W-30 syntec, before I used penzoil 10W-40, I noticed the next time I drove that oil pressure is great (40-50) at start up and idle but once the vehicle reaches running tempurature, after about 20 minutes, the oil pressure will drop to 15 at low engine speeds and come back up to 40 when accelerating and driving. I have checked the fluid level and I don't hear any knocking at this point. Do you have any idea what could be causeing this?

Answer
Hi Cliff,
Aside from the obvious change in viscosity, I know of no reason for the drop.
Of course you could change it again back to what was in it before and see how it acts then.
I will send you a link about different oils that I found. I call it "More than you ever wanted to know about oil".
Makes for some interesting reading.

www.skylinc.net/~kpr/motoroil_faq.htm

Good luck,
Van