I found this topic intruiging, so I did a quick look-see on the Web:
From Kendall's website:
"API SL oils will be the equipment manufacturers recommended crankcase lubrication beginning with the 2002 model year engines. Oils meeting API SL have improved performance over API SJ oils in the areas of volatility, high temperature deposit control and oxidation stability. As part of controlling oil consumption and the effect on the emission control systems, car manufacturers have tightened the limits on allowable volatile material on motor oils. Also to help extend the life of emission control system catalysts, the allowable phosphorous limit in API SL oils has been lowered."
In a Chevron document from 2000, commenting on the upcoming SL standard:
"...This standard will require a significant reduction in volatility and improvement in fuel economy, as well as require fuel economy retention and an improvement in high-temperature
deposit-forming propensity. Though not explicitly part of the new requirements, these changes may
also provide some modest additional margin of safety when left in the crankcase for the full recommended oil change interval, particularly for those who, against good practice, tend not to check their oil level between changes."
These don't directly answer our question (Safe to use SJ in a 2003 car?), but to me it's simple math:
SL oil, 10 qts/yr @ 70 cents/qt after rebate = only $7, the price of a movie ticket, to get some peace-of-mind. Why give it a second thought?