well my motor is rebuilt 68 350 an has 20,000 miles on it should i use synthetic oil an if so what should i go with an what do you guys use?

 

nope, not synthetic - too thin

I use 10/40 dinosaur oil; almost went to 20/50

 

Dude, 10W30 is 10W30 etc. Granted they may be a bit thinner when cold, but at running temps they're the same. That's one of the advantages of synthetic. Don't need the viscosity at cold temps to hold up under running temps. And you might be able to get by using a thinner sythetic oil. Means more hp.

IMO I'd still use an additive though like EOS or similar along with it.
FWIW I've been using straight Mobil 1 for years. I have a roller cam though.

 

We've discussed oils quite a bit. If you use the search function I'm sure you'll find alot of good info. FWIW, I use Rotella T. It is cheap at Sam's Club.

 

With a flat tappet cam, if that's what you have, you need plenty of phosporus and zinc to prevent the cam from getting damaged. Only Amsoil and Royal Purple have the safe amount of those additives for flat tappet cams. If anyone has the specs on any other oils which do, feel free to list the documentation. As of 2009, Mobile 1 and other oils have dropped their levels of zinc/phosphorus in order to comply with federal law. Buyer beware. I now use Royal Purple.

 

Do you have any documentation from a peer reviewed scientific study that concludes that Amsoil and Royal Purple have greater amounts of phosphorous and zinc than other oils? Just wondering...

 

Wonder no more, the amounts of phosphorous and zinc are included on a list of ingredients for each product.

 

I just use Valvoline 40 weight oil in my GTO and change it and the filter often, never have had any problems and that engine is built and not babied. My engine guy approves and he designed the aluminum heads a lot of the old Ponchos use now, been 14 years and it's runs like new. You can worry about some of this stuff too much, I never see or hear of all these problems they talk about, not like you're going to live forever anyway.