My old 289 has about 130K miles on it as far as I can tell. A few years back, my son hit a deer and the animal rolled up under the car and came out the back. Well, his hooves and horns took out the oil pan and the exhaust system. So, I bought a new oil pan and muffler system. So, my gasket for the oil pan is basically new. Anyway, I worried about changing to synthetic because I had also heard horror stories about leaking from the old gaskets. Well, I didn't have zilch for leaking. I recommend you go ahead and change over. Even if you have a little leaking, it will stop after the gasket gets soaked I understand. Mine never leaked a drop though. I use 10W40, but I live in hot Texas. I think most folks use 10W30. These old engines are not as tight as the new ones, so the 5W20 is not a good choice for them. The 5W20 is what I use in my 03 Mach.