Dear Boo;
Multi grade oils were developed for the U.S. Army during WWII as they were unable to service & often in frezing temps w/o 'warm-ups'.
These conditions don't apply today, especialy with custom engine/owners.
The 'detergent action' in most oils offered to the public stops at around 12-1500 miles/60 days.
For this reason(s) I use Dello 400 SAE 30. It is designed for diesel trucks/commercial use and the detergent used lasts around 4000 miles/6 months etc.
I live in California.
Those on the 'East Coast' use multi grade because of the climate.
Good luck, John M...
