What is the best way to clean inside an oil tank ?

I would just use paraffin, pour some in and shake vigorously, rinse and repeat as many times as needed until the paraffin coming out is the same as the paraffin going in.

Same procedure as Bob wrote, but I prefer a degreaser, followed by rinsing with hot water. Maybe you could add a magnet drain plug and one of those modern filters to the oil system. If you think it will have another failure in the future.

The oilway in the sump that returns to the pump - clean it out and check that oil runs freely down it, do not use a wire as you can mistake the change in direction as OK, when it is actually a blockage that you are hitting from either direction.

... after copious rinsing with paraffin, Jizer (my degreaser of choice) etc.

Most of the muck will have settled to the bottom so a jet wash nozzle should get in there OK. Of course you'll probably get several facefuls of the stuff.

In order to steam clean my petrol tank I used a wall paper stripper just leaving the tube inside the tank for an hour as it gaily steamed away. Did a remarkable job cleaning the petrol tank as the sealer (ethinol issue) was coming off in small flakes and getting into carb despite a filter and constantly causing blockages with inevitable results. Obviously dried it out properly and the 99 now runs perfectly (hopefully not tempting fate) cleaned out petrol residues as well as sealer.The tube should fit easily inside the oil tank and you can also use the paraffin as necessary. Hope useful to you and good luck. Hugh

With the new oil filling the tank I remove the return pipe from the tank and run the motor catching the return oil until the oil flows  clean. usually about a 1/4 of a pint.