I believe that is a myth. I have read numerous articles that say you can mix, but you loose the benefit of all synthetic, so why do it. I'm sure others will chime in.

You can mix.....I wouldn't go no 3000 miles on mixed oil like that....

I just today rode around in a car with mixed oil. It'* not like the car will die on the spot. You can call it mixing, but it isn't. When you put synthetic & dino oil in a car, you get just that - apples and oranges. If you put in 2 qt of dino and 3 of synthetic, you'll drive with either 2 or 3 quarts, but never 5. And surely, you can drive a car on 2 to 3 quarts of oil, and it won't break down immediately, but that doesn't make it all right.

So semi synthetic is a mix of synthetic and ...........????

I've actually always wondered about that, but I never looked into it. It really is a synthetic blend with non-synthetic oil. However, it really is a "blend," but pouring normal and syn oil into your engine is truly not blending. That'* as far as I know considering that. We should really have a look at http://bobistheoilguy.com.

All I can tell you is that I switched to full synthetic about 3-4 days ago. After what I've seen the last few days, I don't think I'll be switching back to dino oil...

I think from what I've read, the biggest thing to consider when mixing oils is the additive packages of each oil. Even when mixing two dino oils, if the anti-wear additives are different between the two brands, it'* possible that each additive package isn't good enough when spread over twice the oil that it'* meant for, or that they simply don't work well together. At least that'* one theory that I've heard, which makes enough sense to me that I avoid mixing oils when possible (other than for top-offs).

I would think that synthetic oil would mix with regular though. Probably not chemically blend or anything, but at least intermingle. It'* not like oil and water or anything.

<edit> Oh, and fwiw, I use dino. I just don't put on enough miles to take advantage of extended oil change intervals really. I'm lucky if I put on 3,000 every 3 months. If I was putting on more like 5,000+ miles every quarter though, I'd probably go syn.

NASA, as I was informed, runs synthetic in their giant thing that transports shuttles and rockets. Synthetic oils have been shown to stop most engine wear. It'* pretty cool.

Another consideration I've heard of is that you can't possibly switch to synthetic unless the engine has run that oil all its life. If you rebuild the engine with all new piston seals and gaskets, you should be fine. This may be true for really worn engines that have upwards of 200K or 300K miles, but I switched some cars over cold turkey at about 90K, and they truly have better performance. I'm very satisfied with running all cars on fully synthetic oil.

theJMFC, that'* a pretty cool explanation. Based on that, don't mix Pennzoil with other oils

conventional. thats all a blend is. 20% syn(at best) and rest dyno.

this is just a debate that could go on for eternity. everyone has their own opinions(which they are entitled to). there are so many myths and truths, its hard to sift through.

i hope the original author as well as everyone else takes all these suggestive replies with a grain of salt. soak it in and weigh it out.