I don't think DexRON III is synthetic since Mobil 1 ATF claims to be a synthetic version of Dexron III. The Chevron variety of Dexron III sold at Costco should be fine. I don't notice any difference with Mobil 1 ATF in the SDL. I don't know if it's rated for a longer tour of duty like Mobil 1 engine oil is vs non-sythetics.

Sixto
95 S420
87 300SDL