The Shell Rotella "Syn" is a Group III base stock, which is basically hydrocracked dino oil - not a real synthetic. Mobil-1 (and Amsoil, and Redline) use real Group IV/V synthetic base stocks.
"In a pinch", use Mobil-1 15W-50 oil, which is still commonly available almost everywhere (except Wal-Mart). I still have not seen the new 5W-40 TDT oil on the shelves near me. I'll have to revert to Delvac-1 if I can't find the TDT. I had used the Shell for a little while but I was not impressed... oil consumption increased noticeably past 3-4kmi on my OM617.