I'm not 100 % sure because I think I only know him with shorter hair from later years, but I think that's the late Jarvais Hudson, who also was an atmosphere person on hundreds of shows in the 1970's and 1980's.
oh wow that may be it! I only see one Dallas credit for Jarvais Hudson on IMDB, as "Barnes Supporter" in the campy classic
Po
wer Play. In the episode, two men approach Alan Beam at the fabulous Barnes for Congress roller disco and reach for their checkbooks to make contributions, only for the devious young lawyer/campaign manager to refuse. Neither of the two are the blond mustachioed guy.
HOWEVER, there are not two but
three "Barnes Supporters" in the credits, and... earlier in the episode we have our blond mustachioed man behind the bar in a red jacket serving Alan and Lucy in one of their clandestine romantic dinner getaways. He delivers the only line I can remember him having in the series: "Thank you very much, Mr. Beam.." This would explain why this is his only credited appearance out of probably at least a dozen, (This time, the place is too classy and genteel and devoid of rough and tumble Ewing men to devolve into the barroom brawl he'd had a front-row seat to previously.)
Jarvais is credited as a Gas Station Attendant in the 1978 Lily Tomlin/John Travolta vehicle
Moment By Moment (not to be confused with the later sitcom starring Bobby and James Beaumont and the original, non-Jamie Ewing, "hot blonde" from Three's Company,
Step By Step). The attendant is only on screen for a few seconds, barely getting into the frame, but he does have a line, "here you are", as he hands Lily her change, much as our hero handed Alan his change on Dallas.
A screengrab of the scene:
Is that our guy?!? Sure looks like it! This may be a find on the level (for my silly curiosity, anyway) of
@johnmmil10's astonishing discovery of Pam's Digger's Daughter apartment earlier in this thread!
(Amazon Prime started blocking browser screengrabs a while back, which was my method for getting Dallas images
)
(another tangential observation: Tomlin and Travolta indulge in a younger man/wealthy older woman romance in this film, and.. from my brief sampling, wow, despite scathing reviews at the time, for my money Travolta nails the vibe of the sexy, vulnerable young heartthrob that I'm guessing was the original intention for Christopher Atkins as young Peter Richards!)
(the speedo! the midriff!)
