I have heard this 100 times over. I can’t tell you if there is any validity to it of not. The reason I was give is that Dinosaur oil would create a false seal switching to synthetic would wash away the false seal had create a leak. True or untrue? I have know data either way

Changing to synthentic should not create a leak. All it means is that you had a leak before!!!

You might want to look into getting a set of CMI Etops to let it breath better than the Marine Power Manifolds!! Email me or Ateco Engine and Dyno Shop if interested.

Good luck with your project.

From my understanding the reason you might get leaks on hi mileage engines is that synthetic oil actually cleans up all the sludge that built up over time around the seals. Especially if you didn't change your oil often before using synthetic.

I HAVE NO LEAKS NOW SO I GUESS I WILL NOT HAVE ANY AFTER THE SWITCH. AS FOR THE MANIFOLDS I AM GETTING A DEAL. I AM THE GODFATHER TO HIS GRAND KIDS.

When you switch to the synthetic, change your oil filter again after 1000 or so miles because the synthetic, if you have used conventional oil for a while, will do some serious cleaning and most of the gunk will end up in that filter.

It is definitely time to find another "Mechanic"!! --- Jer

I cnanged my 1991 454 mag anbd drive to Amsoil synthetic and had no leaks. I changed it 2 years ago when the engine and drive had just under 200 hours.
I chnaged my 2002, F350 PSD, at 80,000 miles, with no leaks.
I changed my 1988 softail 2 years ago and it leaks like a sieve. It had 30,000 miles at the time of the switch.

I buy into the theroy that it had a very minor leaks before and the synthetic made it a major one.

I sell synthetic oil to my customers and I have seen some high mile motors go from having very slight leaks to leaking like a sieve. I also experienced this myself on a couple older motors that used rope type crank seals and a few late 80's/early 90's motors too (sbc's,6.2 diesel,chrysler 3.0's and 3.3's). Obviously with the oil being thinner but meeting the same viscosity ratings and the fact it cleans some sludge and dirt it is a possibility. I wouldn't do it,Smitty

Oh yes...it'll leak.
I went through that on my old truck. The first 50K miles was on dino oil then switched to Mobil 1. The valve cover gaskets I had replaced about 3 months prior to switching oils leaked terribly. Regular oil will seep into the tiny gaps where it dries from the heat effectively plugging the gaps. I pulled them out and used Permatex Ultra Copper RTV in place of gaskets. That was the only leak I had and it never leaked again from anyplace.
What I found out, as I replaced the gaskets with RTV, was that the synthetic acts like a solvent breaking down the dried deposits left by the dino oil. The insides of the valve covers had the usual brown varnish left behind by dino oil. The synthetic washed it away from every place it splashed. Even the cast surfaces looked like new, no deposits.
When I bought my boat I switched to Mobil 1. I'm still chasing leaks. Once they're all fixed I know it'll be fine.

I switched to synthetic gear lube in the trans and rear for our old '68 Dodge GTS and now it leaks out the tailshaft of the trans. and out the pinion seal and both axle shafts in the rear. Switched back to conventional lube, too late, it still leaks.