ChicagoGirl: 10w seems a little high for a Chicago-area car. 5w would be better for you, particularly in the winter. Remember, the first number is the cold-flow ability of the oil... winter cold will thicken an oil, so a lower number is well advised. Once it's warm, it's a 30-weight oil, so 5w30 and 10w30 would be similar when warm, and hence shouldn't affect leak rates.
Danjamin - You are correct, cleaning out an older engine can increase leakrates. This is why an engine flush is recommended, and if you notice, they always recommend a regular oil during this process to help the seals swell back up. Obviously, synthetics can clean the engine too, but they don't swell seals as readily as dino-oils do. As for the seal-swelling in the first place, I can attest to this behavior, as I performed an AutoRX and have personally noted a reduction in oil consumption. Many other AutoRX users have noted similar results, too.
kman - not sure why a thicker oil would make more sense in an older engine. Sure, it probably reduces leaks, but it also reduces flow rates, which means you're making the engine work harder due to the lower oil flow. Oil pressure is meaningless if there is no flow, and vice-versa. So, thicker just doesn't make sense to me, but hey, what do I know.
If you're having leak issues, you may want to fix the problem, rather than band-aiding leaks. I strongly support AutoRX, but there are other similar solutions available, too.