Everyone tells me something different, I always used quicksilver on my old baja, but now with twins I want to ensure I am using the proper and best suited oil for the motors. Synthetic or regular and what weights are OK to use? Give me your opinion, I have a 29 outlaw with stock 502 mags.

LOTS of oil posts. Synthetic is way better than conventional. Mobil 1 or Amsoil are most popular.
I use Mobil 1 15w50.
Search the tech section for oil, you'll be reading all night.

I run Mobil Delvac 1300 15-40 awesome oil has all the good stuff in it that they take out for emitions. Diesel oils are great for boat. Cheap too, most engine builders use diesel oils. Run it in my baja with 502

Always been a firm believer of the Merc stuff with my stock 502's. However I did go to to M1 this past time. The M1 oil was actually cheaper at Walmart than the Merc oil. Did have to go to AutoZone for the M1 filter. When the next change comes around, I'll be going back to the Merc. oil.

Mobil Delvac 1300 15w-40

found some Mystic 15-50 for $20 for 2 gallons, says its a synthetic blend for diesel engines and farm machinery. Would this be OK or should I stay away from these types?

I would not run that....for diesel use rotella, or moble 1, merc oil way to $$ for what you get..they don't make it..just have their name and extra $$$ added...royal purple is great oil

I run Mobil 1 15-50 in my 502mpi's. When I swithced to it from conv oil, my oil consumption went down about 50%.
Like was said, less $ than Merc oil whnen you get it at Walmrt in 5 qt jugs.

Same oil recomendations for a 96 454EFI? The Diesel oil idea makes sense, I assume because much like Diesels, boat engines are always "working". I seem to remember Diesel Oils having a higher amount of detergents as well, does this help in the marine application also?

Thanks!
Michael

100% AGREE.... Mobil 1 15-50 and hardly any oil consumption...used to add a quart to each motor every month or so.