A dummy load is very useful to determine if your transceiver is operating like it should, or if the antenna has a problem. I have had one for many years. The first thing I do when I buy a used transceiver is connect it to a dummy load and try all bands for proper operation. I do the same when selling one. I have had mine for many years , identified problems with antennas, amps and transceivers and found it a very useful tool. If the transceiver, or amp loads just fine on a dummy load and won't on your antenna, then you need to check the antenna system. I would suggest that you look at this one for a good value. The wattage rating at peak is only a short period of time say with an amp. You can always reduce the power out on an amp to see if it performs on all bands. As for a 100 watt transceiver you can run full power as long as you want with this one. It's a very good value for the money
has been my experience. http://www.mfjenterprises.com/Product.php?productid=MFJ-264
An oil filled one, in general, has more capacity, but more expensive to ship and handle plus more bulk and weight to move around for testing. MFJ also has those with oil at a reasonable price
if that's what you want.