Not an area I've bothered to look into much before but in theory thinner oils might reduce internal friction enough to affect power and therefore possibly fuel consumption. I've never considered it to be likely that the effects are going to be big enough to be noticeable between say a 10/40 oil and a 5/30 though.
However I was away on business in Aberdeen for a month recently which involves a 550 mile drive up from the London area. I go up there a fair bit and have done for many years so I know exactly what mpg each car tends to give on that route. The current beast, 2001 Focus 2.0 ESP always give 33 mpg. Doesn't seem to matter if I drive a bit faster or slower whereas previous cars have been more speed sensitive in their mpg.
On the way up last time sure enough the two tankfuls were 32.7 and 33.3 mpg averaging exactly what I was expecting. What oil was in the engine when I bought it 12k miles ago I obviously have no idea but it's been topped up with some old 10/40 semi synthetic I had knocking about and yes I know that's A) not the recommended oil and B) I shouldn't have been such a lazy git and should have changed the oil long ago.
Anyway, while I was up there and had the use of a car hoist where I was working I finally got round to changing the oil and filter. A friend found me some fully synthetic AC Delco 5/30 for only a tenner a gallon which seemed like an absolute bargain. Absolutely nothing else was touched, tyre pressures included. On the way back I noticed that the fuel gauge was going down more slowly than expected. I know exactly how many miles it takes to reach each mark on the gauge at a given mpg and something was clearly behaving differently. The first half of the trip was quite normal with little traffic and I filled up in Widnes near Liverpool after 367 miles to find I'd got 35.7 mpg. I drove at my usual speeds and it also took exactly the same 6 hours door to door to get from Aberdeen to Widnes that it taken to get from Widnes to Aberdeen on the way up. I stopped at friends there briefly to collect and deliver stuff on both legs of the trip. The average speed was therefore identical and no obvious reason for the mpg to have altered.
The rest of the trip wasn't so good. Leaving Widnes at 4pm on a Friday afternoon I crawled out of the town in heavy rush hour traffic, got a brief clear run on the M6 and then sat in a bloody accident induced tailback with the aircon on for 45 minutes. After 2 hours I'd only covered 80 miles. Finally I got past Birmingham, the roads cleared and I gave it some serious beans down the M40 because there are no speed cameras there, everyone else drives like lunatics anyway and I'd been on the road for 9 hours already and wanted to get home. So basically the second tank was a mixture of heavy traffic, a solid traffic jam and a 90 plus mph blat for the last 100 miles. None of these conditions ought to have been good for fuel consumption but the gauge was going down even more slowly than the first tank.
I finally got home and before polluting the rest of that tank's mileage with any cold starts and general pottering about I filled up to satisfy my curiosity to find it was a somewhat astonishing 38.0 mpg. The only time I've managed 38 mpg before was an experiment shortly after I bought the car when I drove at 50 mph for 90 miles to see what I could squeeze out of it after noticing that the consumption was generally s**te compared to the Mondeo that preceded it. I've never got even close to that at normal motorway speeds though.
I've had the car for 3 years, done several Aberdeen trips in it at the same mpg and there's nothing else I can attribute the extra mpg to than the oil change. To go from 33 mpg to 36 mpg would be astonishing enough at 10% extra economy but 38 mpg on the same roads is beyond belief. In fact I've almost recovered the cost of the new oil in the petrol savings on one journey. I've worked out that if I'd put in that oil when I bought the car I'd have saved £170 in fuel costs already.
The acid test will be in a couple of weeks time when I go back up there. If the extra economy repeats on the route north then I'll consider it proven. I'll also put the car on the dyno there to see if the power has altered. Last time it was run on the old oil it gave 142.5 bhp so I'm expecting that must surely have gone up if the economy has too. If both factors have changed then I have to conclude that the oil has made a material difference.
To those using cheap, i.e. thick non synthetic oil from your local car parts shop or supermarket it might just turn out to be a very false economy. You could be losing a lot more in fuel costs than the extra cost of a modern thin synthetic oil.
-- Dave Baker Puma Race Engines