MIKE George says:
Rangoon oil was not so much a cleaning agent as a rust preventer. As far as I know it was developed by the British Army, and was certainly in use by troops 150 years ago.

It was a tenacious oil with a very slow evaporation rate, and its purpose was to keep arms free of rust in hot, steamy jungle conditions.

A combination of dampness, heat, and a little honest sweat, is an ideal recipe for rapidly corroding unprotected steel.

Why has it gone out of use? Probably because there are better rust preventers available. And, with internally-chromed barrels and modern metal finishes on the outside, guns are less prone to rust than they used to me.

Don’t confuse Rangoon oil with soluble oil compounds, which are best remembered with the trade name of Young’s 303. Young’s, mixed with water, was once a popular gun cleaning agent, and it is still available if you search around although I have never been very keen on it personally.