i just rebuilt my 351w and wondering which is the best oil and the weight i should use for break in help me out

 

Depends on your bearing clearances. The weight will be the same as the engine was built to run. The type will be an inexpensive conventional oil for the first miles. The total number of which will be determined by how the engine was clearanced. 1500-2000 miles is good for tight engines (10w-40, 10w-30, respectively), 1000 or so would be more than enough for a 20w-50 engine.

 

I just recently broke my 351w in on straight 30 weight with an additive to help the cam break in. They recently removed some elements from motor oil and as i've heard, not using the additive could potentially hurt your cam. Call a performance rebuild shop or comp cams and ask about it.

 

I don't know much about 351s but this doesn't soundquite right. That is some ridiculously heavy oil. Alower winter rating will make yourengine more efficent, it wont have to work as hard.

 

30 weight oil isn't heavy at all. It's actually pretty thin. Don't be confused by thinking 5w-30 is thinner...it is technically very similar, but has enhanced characteristics for its behavior in colder weather.

There is the idea out there that modern oils won't protect a flat tappet cam and will shred it. Haven't really seen this happen, nor have I heard any real stories of it actually happening....just "it could happen" stories. The real scoop would be at the SAE website as to the grade of oil being used.