what weight synthetic oil?

[fe66comet]

I am looking to run Royal Purple HPS in either 10-30 or 1010-40. Traditionally I would use 10-40 in the summer 10-30 winter. But with the lesser breakdown and cooler operating temps of the synthetic should I go 10-30? Thanks FE gurus.

[RLander]

Are you running a flat tappet cam? If so I personnaly would avoid synthetics. A while ago some one published some ZDDP numbers for different oils. I use Castrol GTX 20-50 with a bottle of Red (it has to be red not blue) STP motor oil additive. Year round however it only gets to about 0* here. Anything with flat tappets need lots of ZDDP. Next oil change im switching to 10w-40 however. This is just MHO though. 

[fe66comet]

Roller cam and 340 seat pressure. It gets parked when the first snow hits but then again we got our first snow today but been in the negative F temps all week. I need the synthetic for corrosion protection with using E85 the Royal Purple has a zinc/phosphorus Coating to protect the internals from moisture and dry starts.

[RLander]

I would shoot for 10-40 then. The final viscosity is determined by bearing clearances, the tighter your clearances the thinner your oil should be. Honestly it’s probably out of my league. All I have for you is knowledge I’ve gained from reading alot and forums. Sorry.

[fe66comet]

It would seem logical after thinking about it to run 10-40 due to it will get street use and road track at high RPM.