I heard synthetic engine oil causes gaskets on old cars to leak.?

No, It will not make your gaskets leak! But what will happen is if you already have a pre existing oil leak by adding synthetic oil your oil leak will now be a heavier leak (leak more) due to the molecules of the oil being more "slippery" ADDED: You can thumb it down all you want but it is the proven truth. Lol

it shouldnt. If something it could leak out of the valve gaskets if severly overfull or that's going to run sluggish and burn the oil out the exhaust. the two way that's not stable to overfill. NOw the question is how lots overfull are we conversing. a million/2 qt a million qt you in all probability wont be conscious something even that's not goog. Now 2,3,4 qts over then super issues. 2nd factor is why the oil is over did somebody placed too lots in or is the coolant leaking into the motor. Any way no it is going to no longer reason the pan gasket to leak it doesnt somewhat count selection because of the fact whilst the automobile is off the oil already sits above the place the gasket is any way with the final quantity of oil

Some manufacturers claim the mixing syn with conventional oil will contaminate the oil so I wouldn't recommend mixing the two...

Synthetic oils have been around since the mid 60s and have been used with good success since that time... So I'd have to say that the claim about the gasket leaks is a myth...

It's true. Synthetic can leak past an old gasket more easily than conventional oil. I don't believe you would gain any advantage by adding the castrol 10W30.

synthetic oil molecules are smaller and will find it easier to leak out of the holes in the engine. the oil does not make the leak, only find the weak points easier.

I would say thats a myth. It wont affect the gaskets. I always use full syntetic oils. It could give some performance gain, and improve wear. But not much.

Gaskets on old cars leak anyway

It won't cause them to leak,but if you already have leaking seals it may leak worse.