Aye, the petrol is just beside the pub - you'll be going there anyway
I brought a few big lads frae Scotland tae carry me cans - magic!
The fuel in Tobermory is cheaper than in Oban, so don't be shy. Craighouse/Jura is a bit of a faff for fuel - the keys are at the hotel and it's some way from the pump. Port Askaig, Islay, is a kernycan job too, but the pumps are as close to the water as Tobermory. There is supposed to be fuel on Colonsey, but I'd suggest calling ahead to check. The big Marinas up the west side of the Sound of Jura are diesel only, IIRC. Puffin Divers in Oban have petrol on the pontoon, but expect to pay a 20p per liter premium. Negotiation is not appreciated
There is a pump in Port Ellen, Islay, a shortish walk from the pontoons (250m?)
Petrol is manageable in the Isles, you just need to plan your trips and you WILL need cans, if only for peace of mind and a reserve. A small folding trolley would make life a lot easier if you need to carry fuel. Depart with the cans full, decant when you arrive, take the empties along with you for lunch and return with them filled. It's no biggie...
AVOID the Antler restaurant in Craighouse, Jura. The venison burgers are nicer at the hotel. The staff there are much much nicer too...