I have an original 289 in my 67 mustang.  It has about 65,000 miles.  The car had sat in a Michigan garage since 1974.  The engine runs great and I am adding many of the hp mods.  However I was warned to no use synthetic oil as I was told it would destroy my old seals.  So I have been running Castrol blend, but I would rather go full synthetic if what I heard is untrue.  Any comments  

I've been using full synthetic oil since 1998 and had no problems yet. I buy the less expensive wal-mart brand and always change the oil myself. I use it on all my cars. I've been curious who makes their synthetic oil? I do last longer on oil change intervals. I notice the oil does last longer without that burnt smell compared to the regular oils.  

Hey Tony

I wouldn't jump into putting synthetic oil in your car just YET!
Get all the info you can about what you want to do.

All I know is I'm rebuilding my engine with new cam, new this and that. And what told NOT TO PUT SYNTHETIC OIL INTO MY ENGINE TILL ITS BEEN BROKEN-IN.

Due to the fact that the synthetic will bring leaks if I put that in during the break in period. After the break-in then I can put the synthetic oil in my engine.

However, your engine has mileage on it. Hummm!
Like I said get all the info before you jump in.

Take care
GOD BLESS


Ray    

The reason you shouldn't use synthetic oil during break in is that it's too slick.  The parts need some time to get acquainted and flat tappet cams especially need run in time.  It might also affect ring seat - although if the clinders were honed properly for the rings, they should seat almost immediately.  Flat tappet cams need the lifters to turn so they wear in and syn oil can be too slick to let the lifters rotate.  If the lifters don't rotate, bye-bye cam.  Modern roller  tappet engines do not have these issues.  As for leaks - that's just old BS.  Engines won't leak any more with syn oil than with dino oil.  Maybe if you went from a very worn engine running 40wt to running 0w-20, but I still think not.  

Now, I have heard a couple of folks complain that  syn oil does not stick as well as dino oil to parts on engines that sit a long time - like cars that don't get out much.  However, after trying to wipe up some Royal Purple full syn 75wt off the driveway, I find that a little hard to believe.  Even brake cleaner wouldn't dent that stuff.  

Note to Ray - if your new motor has a flat tappet cam, you should run it in with 15w-40 Chevron Delo (that's right, diesel truck oil) and a pint of EOS additive from the GM dealer.  Zinc is the key additive for metal-to-metal protection and it's going missing or found in very reduced quantities in new oil forumlations.