Track as in "race track"?
30 weight. You don't need to worry about the "W" number because you won't be driving it in the winter.
If you can't find a straight-weight oil (one that doesn't have the "W" in the number), then get one with the "W" number as close as possible to the 30 - e.g., if you have a choice between a 0W-30 and a 10W-30, pick the 10W-30.
Here's why: Most multi-weight oils have additives that thicken the oil when hot (called "viscosity index improvers"). Roughly speaking, if you want a 0W cold rating, you start with a 0 weight oil, then put in the additives to get it to meet the 30 rating when hot. If all you're shooting for is a 10W-30 oil, you can start with an oil that is thicker when cold, so to meet the 30 rating when hot it won't need as much of that VI improver additive. This is important because the VI improver additives are quicker to break down under heat & stress, making the oil less of a lubricant.
This isn't as important if an oil can be formulated without the VI improvers, which some synthetics can do. The bad news is you won't be able to tell how they arrive at their rating by reading the bottle.
Last edited by five7kid; 02-10-2011 at 02:08 PM.