Okay, I'm hopping in here. Many of the stuff being said is accurate, some needs clarification. I sell synthetic oil, so hopefully I'm relatively up to date on what's going on.
The stories about synthetics cleaning seals is true to a point. The PAO base stock and those with Ester do a good job cleaning. Most of the synthetics on the market now though are a Group III petroleum base and do not clean as well unless they have additional additives. Group II oil is your standard petroleum, Group III is what is more commonly known as a hydrocracked petroleum that the API says can be called synthetic, Group IV is a PAO "true synthetic," Group V is everything else which includes Ester based oils. Group I is more or less obsolete and shouldn't be used in today's cars anymore. If the engine has sludge in it one of the things a good synthetic is supposed to do is clean it and keep it clean. But if a seal is bad, it could clean away deposits that were preventing it from leaking. Good news is, the additives to keep seals pliable might restore it enough to seal back up. Also you might notice some oil loss during the initial conversion. This is because the cleaning additives of the synthetic are cleaning the rings as well and as chunks start to come off, you might have blow by. Keep adding oil and when the ring is clean it will reseat.
Should you use a flush? Depends on the flush and how dirty you think your engine is. Some flushes act more like an acid and these can actually damage the engine. I sell Amsoil, and in their case it is a concetration of the cleaning additives in their oil that helps to loosen and remove deposits that won't damage the engine. The oil should clean anything that might be leftover. Should you use it? It's only roughly $4-5 and a clean oil filter to catch what gets loosened, so it isn't too much of an expense. I'd recommend it if you have been using dino oil for a while, but if you've changed that on a regular bases, you probably won't have too many deposits either. But it won't hurt.
I personally wouldn't recommend Redline for street use. Unless they have changed, they are a Group V oil that is Ester based. Excellent for performance and wear protection, but Ester have a tendency to want to attract and hold moisture and they need to use additives to compensate for that. Amsoil and Mobil switched their formulas back in the 70s because of this to a PAO base stock. Both companies still use PAO today. In Amsoil's mid-tier and Series 2000/3000 line they use Esters for additives giving the best of both worlds. Royal Purple, I'm unsure of their base stock now after an article in Muscle Mustangs last year where the RP rep was quoted as saying that all synthetic comes from crude. This is wrong, unless all their synthetic is now the Group III petroleum based oil. Up until then, their lower end oil, common in most stores carrying it, were a blend, and only their high end was a full PAO base. But when comparing specs to other oils, they don't rate as high as they try and make people believe.
Extended Drains: two words that Amsoil coined back in the early 70s that now is becoming more popular. Mobil 1 EP is hyping up their 15K oil, but Amsoil has had 25K oil since 1972. Of course, that is for standard driving. One post commented about the number of high revs, etc. they did. While the oil can be used longer intervals, if you are exposing the oil to these type of conditions, or racing conditions, then oil analysis would be your best choice to determine how well the oil is holding up for your application. One thing to note with Mobil 1 EP oil, they don't warranty the oil for that if your vehicle is still under warranty, only when it's out of warranty. This is their way of avoiding any legal issues with the auto manufacturer should something happen . Amsoil will warranty any vehicle that is in sound condition and follows their recommendations. Most all other synthetics recommend normal drains, so if you run the oil longer and something happens, you are on your own. If you are using one of the Group III synthetics (Valvoline, Castrol Syntec, etc.), you have to remember, these have the same performance drawbacks that any regular petroleum will have. These oils are a bit more stable because of their refining, but you can't change that the base molecule is still petroleum. These oils will last as long as the additives last to protect those molecules from breaking down. This is the reason Amsoil's XL line of oil (a Group III oil) is only rated for 6 months or 7500 miles (more if recommended by the manufacturer). They do, however, use the highest quality stock available for this group of oil, while many others buy the one just high enough to pull it up out of the Group II standard petroelum oils.
Filters: I don't know a lot about the
K&N filters. The Mobil Filters are good and use a synthetic media. The older Amsoil filters used to be the same, but with a longer change intervals, thus the 6 months someone mentioned. Their new Ea Filters (Absolute Efficiency) use nanofiber technology developed for the M1 Tanks. These will filter out 97.8% of all particles down to 15 microns, and to a lesser percentage smaller particles. Many filters out their will says that they filter down to a certain micron, but check to see if they are showing an "absolute" rating or "nominal" rating. Nominal means that they will filter out only some particles down to that size and chances are any good filter can do the same or better. Because of this new nanofiber media, Amsoil now recommends when using these filters with their oil to change them once per year or 25K.
Die hards will say change every 3000 miles, but someone posted that in Europe cars run for 10K between oil changes. This is true, and now is also beginning to take place here in the U.S. GM uses a monitoring system that monitors the breakdown of the oil, not just the milage with a dummy light, and a GM rep in an article I read stated that if a manufacturer made a high quality oil, it could easily be used for 20-30K. Mobil backs this idea up with their Mobil 1 EP. All this only helps to solidify the "Extended Drain" idea that Amsoil has promoted since day one.
Switching between synthetic and petroleum: no issues here, question would be why go back?
Oil Analysis: Blackstone is a good lab to send samples too. Cleveland Tech is another. Amsoil has one that goes under the name of Oil Analyzers, but last I heard these samples end up being shipped then to Cleveland Tech. It runs roughly $20-25 for a good test, less depending on what tests are performed.
Cost Effectiveness: Synthetic has always been said to be too expensive. Has anyone noticed the prices of petroleum oils these days? Some of these are running around $3 per quart. If you can just double the normal oil drain using a good synthetic, you are already breaking even. And if you run it longer, even just three times, you are saving. I just read in one of the e-newsletters I get, Oct. 1 MobilExxon will be raising the prices of their oils again. No word on the other manufacturers yet. Amsoil has also experienced price increases over the past few years, but percentage-wise much less than other oils, especially petroleum with some of them increasing over 100%. Amsoil's prices are roughly the same as Mobil 1, less if you have their Preferred Customer Membership (
e-mail me for details). Walmart always seems to have the best pricing on Mobil 1. The other specialty synthetics like Royal Purple or Redline you'll have to get in stores that carry more specialty products.
Damn, I write too much...