I'm a big believer in synthetics, particularly Amsoil.  My motto has always been: oil is cheap, engines are expensive. 

I too have seen engines where the rocker boxes and lifter valley were both packed full of wax.  Around here it was attributed to Quaker State oil.  I think a big part of it was also that the engines were typically low RPM and likely were used to make a lot of short trips, especially in winter where the oil never reached proper temperature. 

As far as switching to synthetic, on a rebuild, yes, once the break in period is over.  The only danger with synthetic on a fresh engine is that the rings won't seat because the lubricity is so good.  Once the rings have seated there's no reason not to run it.  The only exception is new vehicles that come with synthetic from the factory.  The OEMs have developed proprietary honing processes that get ring seat essentially immediately on first fire up.  In that case you can just continue to run the factory spec oil, or switch to another brand of your choice.

One thing I will add about breaking in a new engine; use a good break in oil.  I really like the Joe Gibbs oil I've been using.  Plenty of ZDDP for flat tappet engines, and even if its a roller there's still benefit to having that ZDDP initially to reduce heat and friction as everything seats in. 

Personally I would put at least a couple hundred miles on it of street driving.  A dyno doesn't really provide a dynamic loading of the engine the way driving does.  Pull some steep hills in high gear, then roll down the other side.  I've always found that was a great way to make sure the rings were fully seated. 

Raymond_B wrote:

Great discussion! @Kristang What's your opinion of the Valvoline VR Racing line? It advertises "high zinc" among other things.

That product is a great option for those with flat tappet cams!  That is my choice for break in oil

I'd stick with the "cheap O'Rielly oil"!  Your engine don't know the difference....for all the driving you will prolly do.
   If you're racing sure......spend the bucks....IF your towing...same answer.
For an 'old-man' putt'in around town and lett'in-er-rip occasionally any SAE oil will do just fine.  Changing it AND the filter on a regular basis (every 6 months or 5000miles) will 'prolly' make it last until you're too old to drive!
I mean..........why?? Less friction will give you a couple extra miles per gallon?! Whoopee.....you drive a modified hot rod, mileage is prolly the LAST thing on your mind.
                                 Enable you to get that last 8 more horse-power when turning 6500 RPM...wooo-hoo... how often (If ever) do you spin it to 6000 much less regular trips to 6500 rpm.
                                  Less friction will enable you to get 300,000 miles before a re-build instead of 250,000 miles..."can-I-get-a-what-what" (I love that!! ....like your gonna put that kinda mileage on a 'bash-mobile'/weekend play-purtty)
                                  Buy what Walmart has on sale and change the filter often. 'It ALL comes outta the same hole' as far as SAE oil is concerned. The rest is just great marketing.  IMHO
                                   Now..........if you are running a flat tappet cam(hydraulic or solid) then a Zinc additive is definitely needed. Buy that when you get the O'Reilly stuff (or Walmart) .....'Outta-the-same-hole'......

Daze wrote:

I will soon have my 351W back together and was planning on running 10W-40 in it like I do in my other Windsor motors but just to be safe I looked it up on line and the "correct" oil for a 95 F150 with a 5.8 is listed as 5W-20.  It said it is important to use this oil because of tighter bearing tolerances.  The bearings I put in it are stock and the same measurements for basically any 351W so why would they be telling me to use thinner oil?  Better fuel economy??  Would love to hear your opinions.

 
Daze, this is from a 1997 F250 5.8L (speed density-non California)

I change my Mustang oil every 6 months  because I don't put enough miles on it  to warrant doing the mileage thang.
I use what ever is on sale. Filter too (except Fram)  It ALL comes outta the same hole guys!
10-30/5-30/10-40..little synthetic/synthetic blend Anything 'greazy'! (well almost)
6sal6