I realize the OP is dated some time ago, but regardless...
Coming from someone who has extensive experience with these types of sites, here are a few suggestions.
First off, YOU have to sell the site yourself. You can not count on your customers (oil companies or visitors) to do ANY of the work for you. Your site is TOO new for that.
You have to be extremely pro-active in building the site, listings included, yourself and obtaining a massive amount of traffic to the site. Traffic, for a site like yours, will be the only true way for you to generate some serious revenue, especially, if you're expecting to be able to sustain the site on monies made from advertising.
The only way any company will be interested in paying you for their listings is if you can provide proof that the site is working and is drawing more customers to their site or offerings.
One change I would make right off the bat is the formatting of the listings pages and how you present the information. Right now, looking at it from my PC, it looks very crammed, confusing and overwhelming.
I would cut down on the information you provide on the listing page, limited to simply name of company, base location (city,state), pricing and phone number. Instead of linking to their site, I would create a page for each listing (and since you are using some sort of CMS, that should not be difficult to do) and link to their "Listing" page. The "listing page" is where the real money will come in...
Within THAT page, you can list as little, or as much, information as you want because it will be a full page for just that 1 company's information.
Within the individual page, you could list just the basic information, to start, and then contact the company personally, asking if they'd like to purchase a "personalized" version of the page which would then include much more information. The personalized versions could consist of a "base" format and then you could offer upgrades and add-ons from there...
But, again, in order to be able to sell something like that, you have to make them WANT to pay for it and the way to do that is to have enough traffic to the site to warrant ANY price (little or great) that you plan to charge.
I have created many sites like the one you have there (directory sites) and I'd be happy to give you a hand with the mechanics of it. If you're interested, send me a PM and let me know.