I went to my local Otter Co-Op farm-store, and bought a brand-new paint can, to re-home the guts of my MFJ-250. The paint-can is internally epoxy-coated. I made a taller vent-tube, by drilling the lid and soldering some copper-coated brake-line. A loose-fitting stainless sheet-metal screw drops into the tubing, allowing for thermal expansion of the oil and air-movement in and out.
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I wanted to address the pervasive oil-leakage out of the original. The epoxy internal coating was the first step; soldering the vent-tube was the second step (MFJ originally used a short, stamped brass vent with plastic cap). The top mounting-surface of the original resistor-bracket received an alcohol-cleaning and thin coating of Permatex Grey RTV, intended to dissuade leakage up and around the SO-239 connector. The underside of the four washers also received a thin RTV coating. Lastly, I cut and fitted 3mm (1/8-inch) silicone foam rope into the top groove of the paint can. When the lid is installed, this rope gasket is somewhat compressed, in an effort to prevent oil-creep here too.
Amazon provided the 8-inch silicone cake-pan, as a final guard against oil leakage. And red is cool :-)
Otter Co-Op also provided the mineral-oil fill for this dummy load, at a reasonable price. Intended as a farm-animal laxative :-)
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I found a few aspects of the MFJ-250 specifications that were "aspirational". Comparing the power-ratings of the Heath HN-31 and HN-31a to those on the MFJ Versaload, I decided to label mine as 200 Watts "Continuous Commercial Service". Good for exciters. And although I had applied 1kW previously to my MFJ, I felt better calling it 800W for 3min after considering all three loads' curves.
(Speaking of "aspirational", we can't forget Ameritron's ADL-1800! Claimed 1500W for 5min, but only using their transformer-oil! You can't get either the ADL-1800 nor their oil ... both now Unobtanium. Their notes warn that exceeding 30s may result in oil expulsion :-O Doubles as a science-fair volcano, I guess :-O )
The DC resistance of my MFJ resistor: 55 ohms. This ought to provide about 1.10 VSWR.
Using my nanoVNA, I also found the frequency / bandwidth spec's to be "aspirational" again. This shows clearly when comparing the Heath frequency range (to 2m) with that of the MFJ.
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Summary: The MFJ-250 makes a great 3.5MHz - 30MHz dummy load, for the usual 100W ham transceiver.
NOTE: There are no longer any Amateur oil-filled dummy loads on the market, following the 2025 demise of MFJ. However, if you own one of these, it will serve for many decades so long as it's limitations are observed. Or, there are the 4-figure BirdRF Termaline products :-)