I'm trying to get an opinion on using synthetic oil for my 1989 454mag with over 500 hours. Teague had reccomended using Amsoil 20w-50 racing as a synthetic for marine but is it a good idea for an older motor that's never had it?

Synthetic is better, period. For a boat that gets very few hours, though, you're changing oil due to combustion impurities, not because of the oil breaking down.

For a stock motor in a low-hour boat, I say go with a good conventional oil and change it often.

For motors that get a lot of hours, or high performance stuff that is stressing the original parts, then synthetic every time.

Synthetic in a flat cam engine is a bad idea. You need a conventional oil with zinc (zddp) in it. If you cheap oil use Shell Rotella. I like Valvoline VR myself.

No disrespect, but you don't have any idea what you're talking about. Yes you need Zinc, but that doesn't mean synthetics don't have it.

+1
One of the highest zinc / phosphorus levels among the commonly available oils is Mobil 1 15W50 (1200 ppm zinc). Most conventional oils now have less than 800 ppm. M1 15W50 is run in huge numbers of flat tappet engines with no issues. I own a few of those engines.

http://www.goldenmotorguard.com/cart/

Very high zinc.

Ken

Yeah, and my AMSOIL Series 2000 Racing Oil had more than that after 60+hours of use.

Sorry to bud in too, BUT. High zinc and Phosphorus helps flat tappet cams..I use amsoil 20-50w. Less engine noise..I was told by several engine builders to stop using conventional oils (like mobil 1,) because of the lesser amount of zinc ,Phos, etc..

The extra cost for amsoil full synthetic is alot cheaper than broken engine parts

Mobil 1 makes lots of oils - some are lower zinc, but 15W50 has plenty of it. So does V-twin, and several oither of thier oils. They publish the levels on the website.

BTW, Mobil 1 is not a conventional (ie, dino) oil, it is a full synthetic.

We recommend AMSOIL 15w-50 DOMINATOR. This oil was specifically designed for a marine application. Many other Synthetic oils are just too slippery and can/will do bad things over time. I say stick to a good straight racing oil OR you wont go wrong with the AMSOIL Dominator or the Mercury Synthetic! -Eric