OK, several myths going around about synthetic oil. First off, let's understand what Synthetic oil is. Keeping it real basic so it's easy to explain and understand: Synthetic oil has had it's molecular structure modified so that the oil behaves like we want it to. What do we want it to do? We want it to flow and lubricate over a range of temperatures. We do not want sludge to form. There's a few other properties we need to get it to do those two things, but they all support those two requirements.

Synthetic oil is better than conventional oil because the basic structure of the oil is good at performing the above two objectives. Conventional oil requires additives to do the same thing. These additives burn off or evaporate over time, so the oil's lubricating properties diminish and it starts forming sludge. Synthetic oil will not deteriorate or form sludge until around the 25,000 mile mark. Conventional oil starts deteriorating the first time you warm up your engine. Conventional oil also is far more affected by temperature than synthetic.

OK, now to clear up some myths:

Why do we change oil? Three reasons. 1. It's lubricating ability deteriorates over time. 2. It forms sludge over time. 3. It gets dirty. So, with conventional oil it's imperative to change the oil every 3000 miles. Even if it is clean enough to remain in use, conventional oil deteriorates and forms sludge quickly. With synthetic oil we only care about the contamination in the oil. That will reach an unacceptable level before reasons 1 or 2 come in to play. When will that happen? Hard running and a dusty environment will accelerate the rate of contamination. Mild running, a clean environment and an oil based filter like a K&N will slow the process. (These filters trap fine dust better than paper elements.)

So how long is long enough? To know for sure, you'd have to do an oil analysis. Too much work, so we change Synthetic before it needs to be changed. Back a long time ago, I had access to oil analysis. I lived in Phoenix at the time, fairly dusty environment. I checked my oil at 10,000 miles once. This was on a supercharged 2.8 that had been run hard and wheeled a lot. The oil was fine. I live in Portland now. Very clean environment. Sometimes I get lazy and don't get around to changing the oil till 13,000 or 14,000 miles. No ill effects noted. As I noted above, my 5.7 might as well be new considering performance, smoke and leakage. It runs like new, it does not smoke, it does not drip. One quart oil use every 10,000 miles is good for a new engine, let alone one with 212,000 fairly hard miles on it.

As far as going back to conventional oil, sure it's bad. Conventional doesn't perform as well as synthetic. But it's no worse than running conventional oil in any engine. Nothing mythical happens when you run synthetic oil that makes switching to conventional oil any worse than starting out with it.

Changing to synthetic is always good, no matter how long conventional oil has been run. Sure, the engine might start to smoke a bit. (Or leak a bit...) Believe it or not, that's a good thing in this case. Why is the engine starting to smoke? It's not because the oil gets into places because it's finer. It get's into places because the synthetic oil is cleaning away sludge that has formed. That sludge has prevented the oil from getting where it needs to go. Now that the sludge is being washed away, oil is getting back to where it needs to go. Yes, the seals have deteriorated. That doesn't mean you should be without oil on those parts. Rather the opposite, you need lubrication more than ever so things don't start wearing at an ever increasing rate.

Is there a problem changing synthetic oil every 3,000 miles? Other than your time and money, of course not. I do think your time and money would be better spent elsewhere though. Certainly 5,000 to 6,000 mile changes should satisfy even the most demanding requirements.

Synthetic/conventional blends. Wow, what a total waste of money. The conventional portion will deteriorate and perform just like conventional oil, because that's what it is. The Synthetic portion just adds cost to the oil. The worst of both worlds. The weakest link determines the strength of a chain, and in this case the conventional oil determines the quality of the blend.

Switching from conventional to synthetic. Always a good thing. However, I'd go with a 2000 to 3000 mile change interval for two or three changes if the rig is high mileage. Why? Simple, the synthetic oil is going to be washing out the considerable dirt and sludge the conventional oil left behind. It's going to get contaminated a lot faster because of this. Your engine is also going to run a lot longer because of this.

If you know much about oil you'll notice that I really simplified the advantages and properties of synthetic oil. I just figured that this post would be more than long enough. Suffice it to say that synthetic is better in every way than conventional oil, except for cost per quart. However, if you change it one third as often, it actually becomes cheaper than conventional oil. (When you include filer cost and the cost of your time.) Conventional oil also deteriorates over time without ANY miles being run. A couple of months or so and you should change it regardless of mileage. Synthetic oil does not deteriorate over reasonable lengths of time. ( A year or two.)

It is ALWAYS better to run synthetic over conventional. Running synthetic oil with 10,000 mile change intervals is FAR superior to running conventional oil with 3000 mile change intervals. And cheaper.