After reading "Can viscosity affect gas mileage?", it appears that it can affect gas mileage.
However, what would be the consequences of going from the manufacturer's recommended 0w-20 to the 5w-40 that fansites often say is "better for tracking" on my car? Will it affect the wear on my engine? How would it affect performance for day-to-day drive?
From the referenced link:
slight decrease in engine noise
I have experimented with this on a Triumph motorcycle engine, 10-w30 to 10w-50. The older models are bit noisy in the valve train and I agree with the above statement. The difference was minor, but present.
Will it affect the wear on my engine?
In theory, if you live in a particularly hot climate, the 0 vs 5 number could closer to what the mfg expected. A very small difference anyway. As for 20 to 40, it won't damage your engine, but I would also agree with a conclusion made in the linked question- the oil pump is working harder. Additional load is not necessarily the same thing as additional wear, however. Heat generated, but not sufficiently dissipated causes wear. Oil pumps are simple and extremely durable components.
It's worth remembering that over the course of, say, 7k miles your oil changes viscosity anyway. First, it thins, then as it passes (7k? Who knows) it will begin to thicken again. Btw, you don't want it to get to the point where it is thickening.
How would it affect performance for day-to-day drive?
If under controlled conditions there was a 0.8% improvement in fuel economy, I would say, no noticeable difference. In theory, a small loss of rev-up acceleration.
I think the best logic I could use to convince someone to use the mfg recommended viscosity is that the mfg tested and designed the engine with that spec in mind. You know you're getting the intended reliability and performance in the product your were sold.
i just put in 5w40 instead of 5w20 in my Japanase Mitsubishi Mirage 1.0L and there is a noticeable loss of acceleration. I'm thinking of draining it and putting in the correct oil.
