Putting any oil into your engine is fine. There really are no proven facts about any of the opinions posted in this thread. All oils are at some levels synthetic anyway, just like all of them have some level of cleaners in them. Sludge clogs up engines on it's own, though certain things accelerate this process (like seafoam) but the sludge already has to be in the engine. If people use modern oil and change it regularly sludge won't form in the first place. The really funny part of all this is that in decades past like the 50s, 60s, and 70s motor oil was crap for the most part and sludged regardless. Quaker State is one that comes to mind for having had losts of waxes and parifins in it when it first came out 50 years ago more so then others at the time. Everything sludged and people produced different cleaners, keep in mind that engines didn't last nearly as long back then, and sludge was a huge concern. Fast forward to today with modern oils (everything on the shelf) with parts of them synthetic, having cleaners in them, and being less likely to break down you just don't have the problems that people still talk about. Cleaners that used to be needed aren't needed anymore, but people still sell them because of the old time way of thinking. Everything has advanced, and unless your car has been outright neglected I seriously doubt that there is any sludge in it what so ever. Like I have stated earlier if you have a sludge problem, the engine has been neglected and there is no easy fix. If you think you can benefit from synthetic then put it in the car, it will free up a few horsepower causing the engine to run more efficently, better gas mileage, and longer lasting oil, how much so is a matter of opinion. To what degree you use synthetic really doesn't matter. I have agreed with some arguements about synthetic helping to get leaks going, but I also think that those leaks would have started eventually anyway. It's my opinion that synthetic doesn't hurt anything in anyway you use it, if there was they'd put a disclaimer on the bottle. If any of these opinions on synthetic could be proved people would have sued somebody by now.