I dont know about Exxon/Mobil, its been years since I did them for base pay, and it been over 2 years since I've done any at all, though it wasn't my choice. I do Shell and Chevron when they are available. But there are never any Shell stations in December. Shell will most likely start up again in the middle of January, and get some bonuses in February. I don’t know how they schedule the Chevron stations, but there always, or almost always seem to be a few available, and sometimes there are a lot of them available.
I check the job board almost every day on both platforms. And then I check the job boards for several other MSCs. And I usually have the bulk of my shops scheduled for the month before the 15th, but I still check for more and try to fill in routes every day.