Syn won't do it, not in my experience.
I switched my first Ranger 2.5 to full syn at around 75K miles. It burned a little more than usual during the first couple of changes, then leveled off to the same amount of oil burning as before (2 qt in 5000 miles or so). No leaks period.
I switched Ranger #2, a 3.0, to full syn at 86K miles. It did the same thing, burning a little more during the first change, then leveling off and only burning 2 qt in 5000 miles or so. It does have a very light film around the rear main seal, but that's typical on 3.0s and it was like that on dino (the film was there when I got it at 49K miles). Aside from the light film, it has no leaks at 97K miles.