Check this out. I called Wal-mart to check out their price on a full synthetic oil change (Mobil 1) and the "everyday low price" is $27.00 Holy shit I say!!! That is a great price for FULL synthetics and an oil filter. That can't be beat!
some suggest a Motor Flush (Made by such companies as Gunk). Its a quart of solvent you pour into your car and run the car for five minutes with it in (don't drive). But you can only usually use that if you're doing the oil change. I'm sure that just dropping it in wouldn't hurt anything. It might be dirty for the first few changes, but other than that, it should be fine.
"After 409,000 miles without an oil change, the AMSOIL-protected Mack E7-400 engine in this truck was as wear-free as an engine in comparable service treated to twenty times the oil changes.
After 630,000 miles total and 409,000 miles without an oil change, the E7-400 engine was torn down by the local Mack dealership in December 1996 and its parts were examined by an engine rater from Lubrizol, a major oil additive manufacturer. " I took all this off of the AMZOIL website.
Its hard to say whether or not the Synthetic will fix your leak. I can tell you that if you use synthetic though, you're bound to get a lot of miles out of your engine. The wear rate is much lower and you'll notice some smoother driving.
note: this is a good question for someone that isn't me :p
there has been alot of talk about this on the Dakota boards...
synthetic will not cause leaks...if you have an existing leak it will find it...also if you have been running dyno for a long time some leaky seals may be sealed up with crud from the dyno, you change to synthetic and you have a leak cause it cleaned all of the crud out of the seal area. Synthetic will also help to keep your seals soft and in better condition as well as high detergent properties. One huge advantage for synthetic is its low temp flow rate...right now if I take a bottle of 5-30 castrol and 10-30 Amsoil in the 20 deg. weather we are haveing and pour them the Amsoil will not be anywhere as thick as the 5-30 castrol...I personally did this last winter for giggles....
when you change a high mileage engine just use 2-3 quarts of synth with the rest dyno AND A QUALITY FILTER . if the oil gets dirty within 1500 miles change the filter only and replace the lost oil with synthetic, at 3000 miles dump the oil and filter and change to full synthetic. Then run your 6-7000 mile oil changes.
Hope I helped a little on the topic
just drain the dyno and throw in the synthetic. for first 2 or so changes will come out alittle dark, after that tho it will be the nice amber color... i dont really suggest mixing dino and synthetic, may have some probs with it, not sure just dosnt sound too good to me, just do a full swap.