Although I haven't seen much discussion on these topics in this newsgroup, in the more popular groups you would be opening a very large can of worms as these topics have been discussed to death.
Here is how I feel about each of these issues. Some will argue one way, some the other, but this falls along the lines of general thinking.
- Synthetic is better than natural oil if you live in a climate where you experience more harsh driving conditions, namely either very hot or very cold. Synthetic flows better when cold (for better starting) and doesn't break down at extremely high temps (for better protection while towing, high speed, in the mountains, etc.). That being said, at a 3,000 mile drain interval you will be changing any oil you use - synthetic or natural - long before you break it down or it fails due to additive depletion. Synthetics may give you a slightly longer drain interval (I change mine at 5,000 miles in my Chevy truck), but you should be considering the capacity of your filter as well, and many stock or stock replacement filters aren't very large. If you want to go past 5,000 mile intervals then you need to change the filter at roughly 3,000 miles and be looking into having your oil tested by a lab to monitor it's ability to keep performing and let you know when engine wear starts to increase. Because we tow our camper with it, I use Mobil-1 in my wife's Sorento with Purolater PureOne filters and change it every 3,000 miles which is overkill, but Kia's don't take much oil and it's inexpensive peace of mind.
If you are wondering about the differences between synthetic motor oils (Mobil-1, Valvoline, Amsoil, Red Line, Royal Purple, etc.) I say use the one you can get easily at a reasonable price. The 10w-30 Mobil-1 I use in the Sorento is readily available at the auto parts store around the corner and at Costco in Orlando, but I have to get the 15w-40 for my truck (it's a diesel) from Amsoil because I can't find it nearby. The incremental differences between the brands are not significant in as much as if you push your motor to the extremes where any synthetic oil will fail then something else in you motor will fail that is not lubricated by your oil first. In other words, at these extremes, the weak link won't be the oil but rather something else.
- Additives like Slick50 are more snake oil than motor oil. Slick50 (among others) has been sued by DuPont for using Teflon in their product and making all sorts of foolish claims. No reputable testing agency - including Consumer Reports if you believe their stuff - hase proven that any of these products come close to doing even a small percentage of what they claim. If the product actually worked, they would be rich beyond their wildest dreams just from the franchise fees the big oil companies would be paying them to license their technology to add to their own motor oils - but they aren't because it just doesn't work. The PTFE (Teflon) these products use just doesn't coat the internal surfaces of the engine the way they claim. At best it coats the bottom of your oil pan, and at worst it clogs up your oil filter. The only true way to pre-lubricate your motor prior to start-up is to use a device called a "Pre-luber", which is essentially a free-standing oil pump that circulates your engine's own oil before you start up you car. There is no substitution and no miracle chemical compound that can do this on it's own.
That's my two cents worth. Remember, any advice - including mine - is worth exactly what you paid for it.
Cheers - Jonathan