You can tie the rib at the Wharf in Oban and walk easily to their Tesco for fuel, if you were stuck.
Try contacting, Oban Yacht Haven, opposite the shore, They may/may not do petrol.
As already mentioned, you can possibly get fuel further South, at Puffin, they also are adjacant to a B and B.
If you head further South, there are excellent overnight Facilities at Craobh Haven, Marina, contact Jim Berry there, he can put you in touch with Lunga , regarding accodomation, also food etc at Lord of the Isles.
There is no petrol here however.
If you venture further South, you can get petrol delivered to the Quayside at Gigha, by Quad, you need to contact Gigha Stores, there is also a Hotel there, and a summertime Quayside Bistro.
I dont know how far you intend to go, again there are new pontoons at Craighouse Harbour on Jura, no Petrol though, a good hotel there, food etc, and if you make it that far South, Port Askaig on Islay has an adjacant petrol pump to the Harbour, 2 mins walk, you would need to contact the Hotel about it. good luck