I would change the oil at 100 miles, then at 1000 miles, while breaking in the engine. My engine builder recommended running 5000 miles on regular organic oil, before changing over to synthetic. Synthetic oil is too slippery for proper piston ring break in. I'm at 2900 miles now. I'll be switching to Mobil 1 at 5K miles.
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Its the same for cam break-ins: synthetic is too slippery to properly break in the cam. the lobes don't get properly worn from what I have been told.
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65 Mustang 289 C4. Edelbrock 600cfm (no more Pony Carb Autolite), Shorty Headers with 2 1/2" collector & glasspacks, Shiftkit, 5-leaf stock height rear springs, 620lbs/1" drop front springs, Gas Adjust shocks, 14x7 American Racing S200 Daisy Rims, low-mid range cam, 1 1/8" front sway bar & 3/4" rear sway bar, & ~3.80's (I think) in the rear.
The reason for using non synthetic oil is to allow a couple of things, the most important of which is to allow the Rings to seat to the cylinder walls. I cant answer what BMW/Volvo/Merc. uses and why but I know this much, Syn. is so slippery that it does not allow the Ductile Iron and Chrome(ium) rings to wear off the imperfections on the cyl. walls and rings resulting in Blow by. The excessive blow by can result in oil consumption that never stops, plugged emission systems and the sort(even though a 66's emissions is a PCV valve and oil breather cap)
The rule I was taught is use comp cams or some other break in additive for flat tappet cams and then conventional oil for 3-5 thousand miles, then put what you want and put your foot through the floorpan.
This is just my opinion and what I do - wrong or right I will keep doing this, it has served me well to date.
Joe
Hi again. That's the reasoning of the auto manufacturers exactly. People don't really ever think about this, but, what does 'let the rings seat' mean? What that means is that, since the cylinder bore and/or the ring grooves are so badly out of round, you have to allow the rings themselves to knock the bumps and flat spots out. The synthetic won't allow that to happen because it lubricates too well. BMW, Mercedes and Landrover ( Not Volvo. If my husband ever pulls into the driveway in a Volvo, it will either be a loaner or something that he will sleep in. ) feel like they can build motors with cylinders and ring grooves close enough to round for that to not be needed.
Regardless of the oil, if you have the original Ford engine and it has been rebuilt with stock cam and lifters, you should use the zinc phosphorus additive now with 50 miles and every time you change the oil. This will save your cam and lifters. The zinc phosphorus will give you a similar oil as that available back in 1966. MotorHeadOil.com has a product called Z-Boost for $9.95. If you have a roller cam & lifters then it will not be as important but will still help.
I've never heard of it. Is it something I can get locally? I've always used Mobil 1 in my newer cars. What is the difference?
Its what they run in diesels. Since they dont have Cats. the anitiwear agents can be added. Usually the antiwear agents will kill the cats., thats why they have stopped adding them to alot of oils.
I don't think I can go wrong with changing the break in oil now at approximately 25 miles and go the conventional oil and new filter for the first 1,000-3000 miles and then change to synthetic at the next oil/filter change. That way I'll know for sure the engine rings are broken in along with the cam. THANKS ALL
i have broke in a hand full of dirt track race engines with new cams and the man who built the motors also grinds his own cams they are called predator cams out of mount carmel pa awesome builder he swears to break a motor in with with penzoil 20-50 in our case and then go syn. and we havent had a motor come apart yet and thats constantly 8200-8600rpm
almost all of todays oils DO NOT have the anti-wear additives they need.(thanks e.p.a.) especially with a flat tappet cam,always add a good engine oil supplement. all of the cam makers have their own, but almost any brand will do. ROYAL PURPLE has one that i use and it works great!
royal purple and xr1
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"After all ......looks ARE important"
1967 Diamond Blue Convertible
all original
Wifey's '67 Coupe Black, GT/CS Clone
I don't think I can go wrong with changing the break in oil now at approximately 25 miles and go the conventional oil and new filter for the first 1,000-3000 miles and then change to synthetic at the next oil/filter change. That way I'll know for sure the engine rings are broken in along with the cam. THANKS ALL
This sounds like the best thing to do.
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65 Mustang 289 C4. Edelbrock 600cfm (no more Pony Carb Autolite), Shorty Headers with 2 1/2" collector & glasspacks, Shiftkit, 5-leaf stock height rear springs, 620lbs/1" drop front springs, Gas Adjust shocks, 14x7 American Racing S200 Daisy Rims, low-mid range cam, 1 1/8" front sway bar & 3/4" rear sway bar, & ~3.80's (I think) in the rear.
That's the reasoning of the auto manufacturers exactly. 
People don't really ever think about this, but, what does 'let the rings seat' mean? What that means is that, since the cylinder bore and/or the ring grooves are so badly out of round, you have to allow the rings themselves to knock the bumps and flat spots out. The synthetic won't allow that to happen because it lubricates too well. BMW, Mercedes and Landrover ( Not Volvo. If my husband ever pulls into the driveway in a Volvo, it will either be a loaner or something that he will sleep in.
