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My mpg went unchanged using either oil.
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That's interesting! Which oils were you using? And, what oil filters?
What kind of car are you discussing here? (engine size/transmission type (auto/man)/mileage on the car at the time of switching over to the synthetic lubes-NOTE: this is of HUGE importance! If you switched to synthetic after about 25,000 miles (a rough rule of thumb), I would totally agree that you are RIGHT in your mpg not changing. That's because significant wear has ALREADY occured in the rings, etc., and the synthetic oil can't undo that. Based on lube oil analysis, the wear that synthetics minimize occurs primarily in the first couple of oil changes (3,000-15,000 miles) from the car being new. That's why I have always changed my cars over to syn-lubes within the first couple of factory oil change periods (normally 3,000-9,000 miles). Also, switching back and forth between petro and syn lubes (discussed in an earlier post by someone else) is a waste of time (and money); pick one or the other (petro or syn) and stay with it! Otherwise, within a short span (25,000-30,000 miles) the wear, while using the petro lubes, wipes out the ability of the syn lubes to keep clearances to a minimum, and therefore leakages to a minimum, and wipes out the syn-lube's ability to minimize wear and keep your gas mileage as high as possible.)
Every time I've switched one of my cars (1984 Jeep Cherokee, 1986 Dodge Caravan, 1999 Dodge Caravan, and now my 2001 PT Limited) to synthetic (after the initial breakin period (usually about 3,000 miles), I have ALWAYS seen at least a 3-4 mpg increase (especially in highway driving). Admittedly, automatics don't show as much an increase due to the transmission's losses (between 25-35% as compared to a manual transmission, that's why the EPA for an automatic is less than the EPA for a manual tranmission.), but my Dodge Caravan (3.3L) got about 27-28 mpg as compared to the EPA (24 mpg). (The ONLY reason I got rid of my 1984 Jeep was because it had a carburator, and it was becoming too hard to find someone that really knew how to tune it up. It was getting between 27-29 mpg when I turned it in for recycling) I also use lube oil analysis to see what my wear rate is, and to verify that the oil is still doing its job. I make my oil (and filter) changes based on the oil analysis. While under the new car warrantee, to keep DC happy, I sample every 3,000 miles. Now that my car is "out of warrantee (35,000 miles)" I'll switch to 5,000 mile samples. The analysis is about $20, but only takes about 10 minutes to do (and I don't have to get under the car, or change clothes to do it!) It costs $1.70 to mail off, and it comes back in a week. I had more, but I guess I've talked too long. Sorry. JL Warren[u]</u>