I would recommend synthetic on newly rebuilt engines just like I recommend STP oil treatment on newly rebuilt engines.

IMO, In a worn engine Syn oil is a waste.

 

I've run Mobil 1 and Castrol Syntec in my '79 Supercab F250 460 for over 150K with no problems - this includes towing a 24' box trailer with tools, a race car and a camper on the bed in 100 degree weather. I also use a water/oil heat exchanger and remote filter. Synthetic does have a somewhat greater tendancy to leak - I think because it does not cause seals to swell as much and also because the cold viscosity is less, it can leak out marginal valve cover gaskets a little easier when the vehicle sits. I work in an engine lab and have seen many engines run on similar test cycles with both non synthetic, and synthetic. The deposits, on the valve guides especially, are much less with synthetic. I would also avoid using the orange Dexcool antifreeze in the 460, and mabe other older Ford engines as well. We had several non leaking 460's on test that were changed over to Dexcool and within hours we had coolant in the oil from what seemed to be a compatibilty problem with the OE manifold gaskets and the Dexcool. I'm curious if anyone else has seen this occur with the Dexcool. It does not seem to be an issue with the newer 4.6L engines etc. though.

 

Isnt that the orange/pink GM coolant? I can see why that would be poisonous to a Ford.

 

Get rid of the flex fan, and replace it with the stock clutch fan. Get a factory fan shroud installed. Synthetic oil cannot hurt, and makes for easier starts in cold weather. ( Check the oil forum ) Pennzoil and Quaker State do not have the best reputation around here. Stay away from DexCool antifrieze like the plague. Just my opinion. Synthetic gear oil may be in order for the tranny, but I'm not sure. Should all add up to everything running a bit cooler.

 

Newly rebuilt yes, but not the best for breakin, especially if you are running a steel cam and flat tappets. I found this out the hard way after destroying a pair of brand new cams in a race engine in the first 10 minutes of operation - and yes the cams had moly breakin paste on them. It's okay once the cam has mated to the lifters, say after the first 1/2 hour of use. Synthetic is probably not an issue with stock cams and lifters if some breakin lube is added to the oil.