There are a million schools of thought about this. #1 on the list is not to use armorall because it has formadahyde and dries out the vinyl even more. I honestly think that now (15-20 years in) it's too late to have a big effect on the condition of a dash. The gauge bump, the low spots to the left and right, and the general down-and-to-the-left stretch are all places that the factory fitting makes the dash weak. If they've already gone to plastic, it's too late. Take it from a guy that's had 4-5 perfect dashes go through his hands. The best one I had sat in an unopened workshop for 8 years.

Having said all that, I had some off the shelf "nano" stuff that the dash seemed to suck up. Anything that will "soften" the dash is what you want. Some people say to do a thick coat of 10W30 or vaseline. I bet those would work (seriously!), but I'm not trying it.