I used to spray the under side of my vehicles with used motor oil. What I found wast that after a while there built up a layer of dirt. Scrape it off with a scraper and there was still rusting going on behind the layer. It appeared that all the oil got absorbed into the dirt with none left to protect the steel. Fluid Film seems to be a good coating that creeps but doesn't drip. It seems to do the job. I think any of the name coatings today probably do what they are supposed to do. When I restore the '72 Lemans Sport a few years ago, It still had the Ziebart sticker from the early '70s when the car was new. The stuff was dried out, cracked and brittle like a poor quality paint. It protected nothing and the car was rusted in all the usual places. The name is still around but I certainly hope they changed their formula as it was a total waste from what I saw after disassembling the car.
Our '97 Olds doesn't see winter and has never been undercoated. The car still looks pretty well like new top and bottom after 125K miles of summer driving. It is always garage kept too. I wouldn't worry too much about our Corvettes but a little under coating sprayed into the frame wouldn't hurt.