10W-30 oils tend to be more shear resistant than 5W-30, so of the two I would use the 10W-30.
Check your owner's manual regarding the transmission. On our minivan it says to select the non-overdrive mode on the automatic transmission when trailer towing.
Heat build up in the automatic transmission is a big concern when towing. ****'s recommendation to use synthetic transmission fluid is a very good one. Oddly enough people often pay far more attention to maintenance of the engine than of the transmission, yet expensive automatic transmission failures are in my experience far more common than are expensive engine failures.
Luckily you don't have any serious mountain ranges to cross between Michigan and Florida, at least not serious in the sense of the Rocky Mountains or Sierra Nevadas.
John
[ June 16, 2004, 11:37 AM: Message edited by: jthorner ]