OK, so she regularly sees (and presumably accepts) 10-12 people per day except for a couple, like the oil man? If so, this isn't so much a guarding issue as it might be an anxiety one..recall how sensitive they are.
What is it about these exceptions that is different than the 10-12 she's OK with? The oil man usually is accompanied by a big, noisy truck/lorrie, with other strong, unusual petro smells. That's the parts that might be anxiety producing for her, it's very unfamiliar from the typical 10-12. You want to make it more typical, so when he comes, leash her, and walk out to the truck, praise her for approaching it and tolerating it as it drives into your driveway. Let her inspect everything, and reward her courage. Same with the groundskeeper. Take her to places with new things and smells and help her get accustomed to them.