I've got a 87' mazda 2.0 engine with 143,000 miles on it (B2000) my last change I went with chevron supreme 10W-40 and ST oil filter (I average 24 MPG in town. 28 MPG highway). Will switching to a 10W-30 increase the MPG at all? I'm open to suggestions, I'm considering 10W-30 high mileage oils, aka castrol/pennzoil..Thanks