I plan to do just that routinely (1 quart synthetic 10W-30) and 3.75 quarts of conventional SuperTech 10W-30) in a new Hyundai Sonata V6 once past the first 3000 mile break-in, though not for the cold starting issue. Here, the summers are often over 100 degress F. daily for five or six weeks, and just shy of 100 degrees F. for much of the rest of the summer season. My reasoning for blending is a little insurance against heat breakdown. A home-brew synthetic blend, if you will. If you buy a commercial synthetic blend, you may not get more than 10% synthetic - and probably a "synthetic" that's really a Group III dino at that. By using the proportion you mentioned, you're getting a 20% concentration and if you use a "real" synthetic such as Mobil 1, you'll get the real deal for probably less out of pocket CA$H than the commercial blend. By the way, I'm neutral in the Group III (hydrogen isomerized "synthetics") vs. Group IV (polymerized PAO "real" synthetics) debate. Also keep in mind that some conventional dino oils (Pennzoil, for instance) use both Group II and Group III base stocks already, though I have no idea at what proportion, and might be worth a look as a total fill.