Oil vicosity breakdown

[Broken_down?]

OK here is the deal.. Oil pressure at full hits half way. which is fine. going down highway same thing. but after a few hours the guage drops to the 3 quarter mark just bfore the low mark. and when I slow down it drop to low. When I go about the day and come back a few hours later and check the oil it is full according to the full mark on stick. I atest this to oil viscosity breakdown. Once the oil cools down it gets it's thinknes back again. If this is the case how do I keep it from breaking down or atleast making it stay thicker longer? I use 30W only. Mobile. I have put valvaline in a few weeks ago. do you think this makes any difference? And anyone selling a dwell meter? or a good set of fog lamps round perfered? Also would a better oil pump help? if so, which one and how high pressure? Any help is apperiated very much.

Thx
Rich.