I alway's figured there was more pressure than that even with an older motor.Shouldn't it be around 40? Or at least 20 with a wore motor.I've had em down to 5 lbs at idle but they were pretty wore out.I'd think that a clogged filter would increase the pressure a tad.Also a quart low could cause less pressure if by chance it was drawing air bubbles?Also mabe a peice of dirt got into the relief valve.I'd change out the oil and filter and see what happen's before overthinking it much more.