I'm looking at Mobil1 15w50 for my Nissan 300ZX. Curious if I used 3qts 15w50 and topped it off with M1 extended performance 0w20.
No such animal as 0W20 EP.
14.95 cSt at 3/1 15W50/5W20EP
14.85 cSt at 3/1 15W50/0W20AFE
Basically is would be similar to a 40 Weight HDE0. Have you considered just buying a gallon of Shell Rotella T6 5W40 ($21) if you want a heavier 40 weight (compared to M1 0W40)?
Personally if you havnt tried it I think M1 0W40 would make an outstanding oil for that engine.