Since I've been using and selling AMSOIL for over 25 years, you are welcome to consider this as biased:

I would not, and don't, run any engine on other than synthetic. I do extend the drain interval. I wouldn't use other than Group IV (or higher) base stocks, which eliminates Castrol Syntek.

I strongly recommend using a good engine flush, such as made by AMSOIL or BG, before changing. If you don't, or won't, then go ahead and put in the synthetic. Watch the oil level--if usage suddenly increases, change the oil filter and top off. If you don't flush, you might want to drop the first load after 3-4,000 miles, as the synthetic will clean out whatever gunk and/or sludge has been left behind in the engine by non-synthetics.

Unless you are going to go 10K miles between changes, Syn is not worth the extra cost. I change every 3K miles and use Valvoline. But I was brought up with the thinking that oil changes are the cheapest preventative maint you can do. I also do a tranny (auto) flush every year, about 8 - 11K miles.

Use syntheic oil. Mobil 1 or Amsoil are best, followed by castrol Syth ATX. The weight depends on where you live, but generally if it's someplace that is temperate and gets pretty cold in the winter, use 10w30 or so. Synthetic oil don't break down easily as high tempratures like petrolium based oils do, and lasts much longer (saves the extra money you spend, plus offers better protection). Popular belief is that using synthetic oil in a high mileage engine will cause the seals to leak. Popular belief is also that if you change your oil every 3k non-sythetic oils are just as good. This is not so, Synthetic oils are engineered from scratch in a lab to provide the best performance and protection, whereas non-synthetic oils are only as good as it is coming out of the ground. Even if you change your oil every 3k, sythetic will still provide you with better protection and operation for those 3 k. This is not true, most high quaility synthetic oils actually are designed to rejuvinate and/or preserve exsiting seals. Just my two cents, always used sythetic Mobil 1 in all my cars, and I've never had a problem in that manner =)

BTW, Wal-Mart is a good place to get Mobil 1 for a good price.

Brandon
95 854T 92k