I just had a long thread on this but all my sources ran dry for other brands this is all i can get locally 440 rebuild 375hp no crazy cam and springs just flat tappets oil opinion please thanks!

Royal Purple is fine but that's just a small part of the break in process. You may already know this but if not these are a few things I used to to when I built engines with flat tappets:

1. Brush moly on the lobes and on the lifter faces

2. Make sure the engine is READY to start on the first few rotations

3. Once the engine is running, rev the engine from 1000 rpm to 2500 rpm several times for 5 to 8 minutes to make sure all of the lifters are spinning

4. After 30 minutes of run time change the oil

5 Cut open oil filter and inspect the folds in the filter element for large metal particles and flakes

6 If you start hearing a loud ticking sound between steps 3 and 4, remove cam with destroyed lobe or lobes, replace all bearings, rings, preform light hone on the cylinders and buy a hyd roller and never look back.

You would be fine with some 15W40 and a can of either Comp or Lucas breakin additive.

We use this combo when we freshen the 540 in the Duster and we run a very aggressive flat tappet with fairly high spring pressure. Never have a problem