Problem is, if you have already oiled it up with
weapons oil, you will need to purge the oil out of the dirt with a
good long boil in water with either
Gringott's or dishwashing soap. The DW soaks won't be as effective with the oil remaining in the dirt, if it penetrated as it often does.
Weapons oil is designed to penetrate.
If you want to try something that might
work for you, get some CLP Breakfree, it is also a
weapons oil, but the C in CLP stands for "clean" (and the others, "lubrcate, protect"). The upside is that CLP slowly but surely penetrates, is somewhat
good at breaking down common iron rusts, and will really get in the dirt and slowly loosen it. The downside is that CLP leaves behind a microscopic teflon coating as the oil's carriers evaporate over time, which is easily removed by further cleaning, but may not be desirable if you want to go with water soaks later on.
CLP should be available in any gun store or online, the
military specified it for howitzers years ago and uses it on all small arms now, and it was commercialized and is successful at least here in the US. It is an awesome rust inhibitor too, if you have anything metallic that is prone to rust.