I have heard different opinions, but I thought I'd ask the pros............."the pros" being you out in EC world.
Can Lucus top oil and synthetic oil be mixed together? If not which would you recommend for 66 Elky with newer 350? Synthetic or lighter weight motor oil (like 10/30 or 40)combined with the top oil?
And finally, does synthetic oil need to be changed as often as your standard oil?

Never tried any of the synthetic's in mine. I used Valvoline in all my cars back in the early 70's, in 75 I switched over to Castrol GTX 10W40 year round. I'm still running it to this day. Every time I've had to open up the engine for work it has been sludge free on the inside. I have a friend who uses Slick 50 everytime he changes his oil, as a result of this he had lower rpm's and less wear and tear on his engine.

For all out protection synthetic is the best you can put into an engine. If your engine has been newly rebuilt make sure you get it broke in first and then after the first few hundred miles change over. If you look around the internet you will find many articles showing positive test results on synthetic oil. It came as standard on my 93 Corvette and is in most high end sports cars. It will last much longer than conventional oil and does not need to be changed as often I would not hesitate to run it for at least 6K miles. If the price concerns you try the blended oils which combine synthetic and conventional they give you the best of both. I too have used Valvoline for sometime and now use their synthetic blend in my El Camino, but it is pure Mobil 1 for the Corvette.

I use Mobil 1 in all my rides, never had a problem, never let me down. I've seen an engine that had 160k on it with Mobil 1 (Mazda pickup-4cyl) and when we took the valve cover off it looked brand new, I couldn't believe it. 8O

As far as break in, I agree with Harold K.

I run Castrol Syn-tach in my Nissan. Great stuff. I run blend in the Elky because of high miles on the rebuild. I am starting to rebuild another 350 so I will use Syn. when the time comes.