Powder recommendations for .38 special revolver loads.

[DannyT]

Do not rely on it. Use proven and tested loads from a manual.

Got it, thanks.

[physeptone]

As has been said already, use the longer (357 magnum) cases instead to avoid carbon rings.

I don't shoot 38/357 but shoot lots of 44 downloaded to 44spl in 44 mag cases (basically the same thing as your 38spl/357mag query). I have never had an issue.
I did once many years ago (in the UK) have problem with a Unique loaded round - I now realise this was most likely down to the powder, and i now no longer shoot unique. It was dirty as hell anyway. Avoid unique IMO.

I currently use Titegroup, and would strongly recommend that you consider that. Probably somewhere in the high 4gr range should be where you will want to be for 158gr projectile.... (obviously used published data, not my guess!). It should be perfect for your needs once you fine tune it.
It is not position sensitive and doesn't mind small quantities in large cases (I use 10gr in my 45-70 rounds, in those big cases)