I used to run Mobil 1 0W30 in my '99 Civic Si, although it required 5W30-dino (for wear and economy since I ran it pretty hard). I was told that the synthetic equivalent of Zero-Weight synthetic is similar to 5-weight petroleum-based. I had to be careful with it -- since it was so light, I lost a lot between changes -- so much in fact, the ECU shut-down the VTEC system on me during an autocross cornering maneuver (since it's oil-pressure based). The rev-limiter cut way early -- I thought I broke something, took it to the dealer closeby, added some oil, reset the code, and I kept a better eye on things. After 2000 miles, you don't think to check the dipstick, but I should have.
Basically, check your oil level often if you to 0W.
don't know if this is true, but it might be an option for those concerned about going all out to 0w20.
you could do a 5w30 - 0w20 mix.
mixing different weight oil does nothing more than give you a hybrid weight. nothing to worry about. (source)
years ago I asked an oil expert on the Compuserve CARS forum if there was a downside in mixing different weights of Mobil 1 to arrive at something approaching a 15W40. He stated my mix would be close to this weight in characteristics. So that is what I do. (source)
...although i've selected supporting claims. if you do a google search, you will find opinions on both sides of the fence. facts on the other hand (or official information) are harder to find.
that said, i've always read that synthetics are fully compatible with other synthetics and mineral based oils. with a broad claim like that, it seems logical that you can mix weights also.
AMSOIL ASL is compatible with conventional and synthetic motor oils. Mixing AMSOIL motor oils with other oils, however, will shorten the oil life expectancy and reduce the performance benefits. (source)
the last sentence makes sense, if you're mixing syn with mineral.
we've all probably mixed syn & mineral of differing weights at some point. i have, in all the vehicles i've owned - usually because i'm short a liter or maybe half at oil change time, or topping up between oil changes with something else.
i'll probably try a 0w20 mix at my next change (spring, unfortunately - this would really help now).
Lets all get this straight. 0w30 protects at the same temp as 5w30 and 10w30, the first number just says how the oil performs in cold temps while the second number is the protection number tested at certain operating temps. Similar theory to what Metro said in the synthetic oil thread about only really helping in the cold. Sooooo, in colder areas a lighter first number could help, where in warmer climates in summer time, as long as the second number stays constant it shouldn't make any difference whatsoever. And weight is weight the numbers mean the same thing for synthetics as they do dino's the only possible explanation could be the guy knew the exact weight (legally oils fall in to a range so a 5w30 could actually be a 5.5w32 for instance, i'm not sure on what the range is off the top of my head.) Google Bob is the oil guy and start reading.
Haha. I'm putting some 5w20 in my friend's civic this weekend because I saw it for sale and figured WTF. I had something important to tell you metro, but now I forget, and now I remembered again. You're site is number one when you search "block heater mpg" on goodle.