Dear Johnny;
Your right, stay with the standard grade (SAE 30)unless your in a 'freezing' climate etc.
Multi-grade was developed by the U.S. Army in the 40s for use (worldwide) so they would need only one type of oil. It had higher detergent agents also, because there wasn't time for "regular" servicing.
The Shell 'Rotella' is for commercial usage like the Standard Oil equivelent Dello, which I use. Also; the additives last longer (than regular brands) too.
Happy trails.
