Should I use synthetic oil?
Q. Should I use synthetic motor oil? Can I change synthetic oil at longer intervals?
A. The answer to these questions is really beyond the scope of this FAQ. I would direct you to Ed Hackett's excellent oil FAQ, posted irregularly on the Usenet newsgroups rec.motorcycles and rec.autos.tech.
There has been quite a bit of discussion on the list regarding Doug Lewis' opinion that synthetic oil should NOT be used in the SHO engine. Check the archives if you'd like to read about that. Suffice it to say that unless you're using an oil specifically formulated for extended use (synthetic or not), your best policy with regard to engine oil is frequent scheduled (around 3000 miles) changes.
One note: Slick 50, Duralube, and other oil additives are unnecessary with modern engine oils and are likely to do more harm than good according to studies by the University of Nevada Desert Research Center, DuPont Chemical Company, NASA, and lots of other reputable sources. Here's more info.
Q. What about Castrol Syntec 5W-50?
A. From Gary Morrell:
5W-50 is an awfully wide viscosity spread, even for a synthetic. Castrol must be using a large amount of viscosity modifiers to get this spread. Viscosity modifiers (VMs) are long chain polymers that are added to a light base oil to prevent it from thinning excessively as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as the "xW" number indicates. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. Unfortunately, VMs will shear over time, reducing their effectiveness, and can burn and form deposits that cause ring sticking. Most manufacturers have shied away from oils with more than a 30 point viscosity spread. Notice how hard it is to find 10W-40 anymore? For awhile, GM threatened to void warranties if 10W-40 was used.
Try to keep the viscosity spread under 30 points, this will keep your bearings happy.
AMSOIL uses no VMs in their 10W-30 and 15W-40 weights, and Mobil 1 formulates their 5W-30 and 10W-30 without VMs. Oils that start out with heavier base stocks, such as 15W-50 and 20W-50 need very small amounts of VMs, however, I can't recommend either of these two oils if your wintertime temperatures go below 30 deg. F.