::Multiple Triumph owner enters the room::
I know from oil.
viscosity. This is a measure of how thick (honey-like) the oil is. Lower numbers are thinner, higher numbers are thicker. Viscosity is temperature dependent; the higher the temperature, the thinner the oil gets. This works with oil being heated in a skillet.
Viscosity ratings. This is the 10W-30 or 5W-30, etc. This gives the range of viscocities. 10W-30, for example, gets as thin as a 10 and starts off thick as a 30. (Scale is relative)
What it means. In cold climates, you want to start with a thinner oil (like 5W-30). Your cold engine will have a tough time trying to turn over and mix a thick oil. In hot climates, you can deal with a thicker oil (10W-30). An oil too thin will pour down parts too quickly.
In South Carolina, it ain’t no big deal. 10W-30 or 10W-40 is pretty much fine. If your service station uses 10W-30 and you top it off with 10W-40, no oil police will show up at your door. Yes, you should use the recommended oil, but you don’t risk engine siezure or anything.
One thing you want to aviod is switching between detergent free and high detergent oils. This is usually not a problem, in that all xxW-yy and SAE oils have detergents in them.
Contrary to Handy’s post, I truly believe in the 3,000 mi oil changes, for two reasons. First, and of lesser importance (especially in your case), it helps with personal resale. Johnny Car-buyer will think twice about buying your car if you tell him you change oil every 5,000 mi. Second, in more metropolitan areas, there are gobs of things in the air that get into your engine and eventually into the oil (especially in the summer). These things (dusts not filtered out by air filter) can cause a lot of wear inside the cylinders. And heck, it ain’t that expensive to get an oil change.