I know you know this, but comparing M1 5-30 to Rotella 5-40 is apples to oranges. The Rotella is all around thicker oil. In a stock short block equipped car, the only thing the Rotella is going to do is cause extra parasitic losses and if something happens with it in the engine, possibly void warranty due to (please note the sarcasm, because I hate dealerships too) excessive oil pump stress, lowering oil flow and increasing system pressure. (end sarcasm) A built short block done the right way will have the intended oil thickness and intent of use in mind when being built, and the builder, if they are worth their salt, will build this a little on the loose side so that the oil pressure isn't too high from tight clearances when using high viscosity oils.
Additionally, there are many other factors than just minor oil quality differences that can cause a spun bearing. The extra shock load on a bearing due to detonation, overrevs, or even extremely high power on an engine not built for such, such as a little extra rod bearing clearance to allow for a thicker oil film protecting the rod bearing from the forces beating on it.
In short, on an engine with bolt-ons and a tune, the change from 5-20 to 5-30 may be a good idea for peace of mind. DEFINITELY install a real oil pressure gauge and get the real story. If the oil you put in pushes oil pressure too high, it can be just as bad as too little oil pressure.
2013 Focus SE 5M Daily Driver
2003 Mustang GT V8/Auto Wife's Daily
1986 Mustang GT V8/5M Toy
- Jody -