[janoskiss (Steve H)]

8" Dob of EPs: good plossls or related improved designs, e.g., Orion Ultrascopic, Parks Gold Series. A 15mm would be a good all-rounder for your scope. Televue Plossls are highly regarded though I prefer Parks GS and the like. In Dobs at least, the TV Plossls vignette somewhat, especially when barlowed. Also the colour is bit too warm for my taste.

If you want cheap as chips but good 20mm GSO plossl is decent and will also give you a usable high magnification when 2x barlowed. (15mm is not so good in that series: narrower below-spec FOV, but last I checked one out was ~7 years ago.)

The 2mm exit pupil rule is good, but it breaks down for very large scopes. Your scope is borderline getting too big in aperture for 2mm ex.pup. to be your default DSO EP (depends on what targets you're most interested in though). You're looking at a moderately high ~165x magnification, where many of the larger DSOs will not fit in the FOV (unless you get an ultra- or hyper-wide, but that won't be no 8" Dob of EPs).