Re: synthetics and leaks:
I've found quite the opposite. I put synthetic oil in the "old" chevy work van -- 297,000 miles, used about a quart every other tank of fuel. After I filled up with synthetic, oil consumption dropped to zero in 3000 miles. Most of the oil seems to be going down the valve guides -- blows some blue smoke on startup and occasionally needs some "help" starting hot (staring fluid).
I put synthetic in the 300TE last year, and oil consumption dropped quite a bit. I've not tracked very well, since it is my mother's car, but I've only added one quart since, she has put in one, I think.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!