Okay, so I'm not sure if I created this topic before... I'm using all of my F30T12 lamps on F40T12 ballasts, all of which are meant to run F40 and F34T12
four footer lamps. Is it okay to run F30T12 lamps on F40T12 ballasts? More specifically, the HPF 0.80amp ballasts... I've heard so far that F30T12 lamps
do just fine on F40T12 ballasts, even after years of usage.
I bring up this topic mainly because I've had to replace a ballast just now, due to lack of filament heating on one of the ends. (The culprit of why lamps
took so long to fire up successfully.) I replaced that ballast with a... PCB containing HPF R.S ballast, meant for F40T12 lamps only.
All ballasts in use, including the "F40T12 only" ballast, are the green and yellow style 0.80amp ballasts. (Some are Thermo-Matic and Thermo-Matic X.)
Also worth noting... some of those lamps are 25w SuperSavers and Watt-Misers.

Somebody else will be more knowledgeable than I am, but my hunch is you're probably all right with full-wattage F30T12s on a ballast rated for F34T12.  I believe the specs for the two lamps are very similar.  Some F40 ballasts such as the Advance Mark III R-2S40-1-TP do list F30, though in this case with a footnote that it's only been approved by CSA and not UL.  But I wouldn't be concerned about operating them on such a ballasts, especially in something like an open strip light that allows heat to dissipate.  I'd be a little more leery about using the 25W T12s at all or even the 30W on the 40W-only ballast.  Is the 40W ballast NOS at least?  I wouldn't want to stress a well-used one for sure.  On a side note, I still have extra used "energy saving" (0.7x amp) Advance and MagneTek F40/F34 ballasts if you or anyone else could use some.

@Patrick, the "F40T12 only" ballast is unfortunately well used, so much to the point where the sticker has browned out a good amount. I absolutely refuse to
use the 25w lamps on that ballast, but I'm currently giving the F30T12's a try for now, temporarily, until I receive the additional 446-LR-TC-P ballast
I'm about to purchase on Ebay. Right now I've got two of my 11 fixtures, using 25w lamps. I'm of course keeping an eye on the ballast temperature, but so
far they seem to be holding up well. Three of my fixtures contain one 25w, while the rest are all using F30T12's. Minus the constant swirling, and the one
ballast that had filament heating issues, all of them seem to be holding up well as of now.
I've heard that using 25w T12's on F40T12 ballasts is similar to using F20T12 on F40T12's. Is this true?

I'm pretty sure one of my 2xF30 fixtures even came with a F40/F34 ballast.
I personally would be fine with running F30's on a modern F40 ballast (but deff not a vintage one)

Allot of modern 2xF40 ballasts are fine running just a single F40 using the red & blue wires (leaving yellows disconnected).
I actually have one running a pair of F20's..which to the ballast is basically the same as a single F40..either way it "sees" 4-feet of lamp.

Yeah, I think a modern post PCB era F40 ballast that also lists F34 lamps should be fine running F30 lamps. Interestingly for the 2x F40 HPF ballasts, the American version will only list F40 and F34 lamps while the Canadian version with the same part number will also list F30 lamps in addition.