None of the SR-104s I've used have ever had sloshing, that is not normal, brand new SR-104s do not slosh, my SR-104 does not slosh. While the oil is present for thermal considerations, imparting a large thermal mass to slow down temperature variations, the lack of 100% oil will not directly affect the resistors themselves. If the power levels are kept to a normal minimal amount, say less than 100mW as is the usual procedure, there will be no danger to the resistors. If you have a leaky SR-104 and it keeps leaking, the only alternative is to find the leak and seal it, however if you observe normal operating procedures, you will not cause any harm or change to the resistors, just expect them to react to temperature changes quicker than normal. You will have to give the SR-104 some time to stablize at whatever power level you're using since there will be reduced time lag. The lack of full oil will not affect the resistor's characteristics as such, just how fast they respond to temperature and power.
The main question is why is it leaking and what caused the leak? If it was caused by dropping for instance, there is always the possibility that something inside has been affected, including one or more resistors depending on how hard the jolt was. Only time and measurements will tell.