I must say I had the best experience that anyone I ever heard had! I told many people my story but they don't believe it. So I thought I will share it here too:
I own now my 3rd MK1 93-96 Ford Mondeo 2.0 stationwagon with 136HP. My first 2 cars had a mileage of around 9-10liters/100km (23.6mpg - 26.3mpg) depending on weather, tires. I usually drive full throttle and 140km/h (88mph) on the freeway. I ALWAYS calculated the mileage, trough the whole life of those cars.
Now I bought my 3rd Mondeo, a 1996 model with higher gears (there where 2 setups). I got now around 4500RPM (can't remember it right) at 140km/h (88mph), that's very high!
Anyway, since I bought that car with 152'000km last year it was taking constant 10.5l (22.5mpg!) I was really frustrated! I first thought it's because the high gears since the 2 others had much lower RPM on the freeway. I changed tires, the winter came, the winter left, nothing changed on the mileage! It even got sometimes worst, but never below that!
I changed the oil from the previous owner (by the way, the service-book shows all inspections, the car was mantained very good, always got the oil changed etc.). I bought the oil at a Ford garage that is supposed to be used on the Ford Mondeo 2.0 and they gave me Ford Formula (I think 5W-30). I paid 13$/quart, the price almost killed me!
Also changed the oil filter to the Ford filter. The mileage didn't change at all!
Now I bought Castrol High Mileage 10W40 and when I first got gas I thought I did something wrong! Now I drove about 5'000km (3000miles) with this oil and I still got amazing 8.5-9l/100km (27.8mpg - 26.3mpg)!!! I still drive full throttle with that car, didn't change anything except some noname cheap Oilfilter and the Castrol oil!
What is also amazing: I got a water temperatur gauge, that got a bottom line, then NORMAL is written and then comes a top line.
On the 2 Mondeo's before and also on this one with the oil before the gauge would usually show on the O-R when driving on a hot day on the freeway. In a cold winter it would go down to N. But now I am waaaay below N, I am just on the bottom line! That means the temperatur is just right on the minimum of being normal! I can drive also on a real hot day, it won't climb! Just in traffic it will, but that's another issue.
I think engine creats MUCH less friction, and to me that just can be the oil change! I don’t know what it did, I just know I am saving lots of money now with gas since the gas here costs a fortune!
My 0.02cents to this thread…