Our Honda used the Syntec Blend the first 40K, or so. I now blend GTX and Syntec. It's cheaper and gives me a known mix percentage (Castrol won't divulge the percentage of their blend). The Honda 3.0L V6 takes 5 qts. (with an oversized filter) and I mix one of Syntec and four GTX, both 5W-30, for about a 20 percent syn blend. I have had good UOAs on both my blend and the "official" Castrol Syntec Blend (which is probably virtually the same thing). I doubt your leaks are due to synthetic oil, especially Castrol, which is a Group III. I'm doubtful any high mileage oil will help, but once in a while it does soften up a seal that got hard. It can't put rubber back on a worn out seal lip, though, or repair an improperly installed seal or fix a break in a bead of RTV.
