[Kracka]

Any major synthetic is good for a year and can do 15-20k miles in a "normal" car during "normal" driving as long as you use quality gas a good filter (probably smart to chance the filter at the halfway point though). Purolator actually touts the high contaminant capacity of their PureOne gold filter ($4 at Fleet Farm; this is the great filter I mentioned earlier).

You can switch to synthetic whenever you want, but there is a chance you'll burn more oil and/or the detergents in it can clean away sludge that's otherwise plugging leaky gaskets. For a beater, I'd do exactly what Brandon does; Supertech oil/filter every 5-7k miles.