Re: Marvel Mystery Oil
Personally, I wouldn't use anything in it. But whoever stated that newer engine's don't sludge up is wrong.... I've got the LS1 from my sisters 98 Trans Am on an engine stand right now. Low compression on #3 after 160K BABIED miles and oil changes every 3000 miles. The low compression was from the engine being completely worn out. When we pulled it and started tearing it down, I couldn't believe how sludged up it was. Even the valve covers needed some hot tank treatment to come clean, lifters where not worth trying to clean, push rods where gummy, bottom of the pan had pounds of crap stuck to it, etc, etc..
I'm not sure where to point the blame for the problems... LS1's should run an easy 200K from what I've read, the oil has always been Valvoline 5W30 dino oil, the filters are usually Delco's or Wix. Old age and Valvoline is what I assume went wrong... or maybe its a GM thing (never been a fan of them).
Anyways... yes, they do sludge up. Frequent oil changes are key to preventing it, but I tend to think that a high-end synthetic; such as Royal Purple, should be used. That's my recommendation for the OP, change the oil & use a high quality synthetic and a quality oil filter.
If she would have went for the Mustang instead of the T/A like I told her too, my back and feet wouldn't be hurting like they do right now. 