valvoline oil could be a cause. Try some Penrite every day 25w60 (the thick shit). Also could be the oil switch on its way out... try a new one. And what oil filter are you running?? If you are using a crappy (valvoline is in this category) oil filter, you will have dramas. Try a Ryco, or genuine Toyota item (I dont think they are much dearer either).
The last thing it is likely to be is, of course, engine wear, including pump and bearing wear. An unlikely culprit is a leaky pressure relief valve.
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