I have owned one of these for quire a few years, and it works very well! I have light mineral oil in mine for bearings. One trick I learned when using thee pen is; you can vary the size of the oil drops by changing the amount of time you press the button and have the tip open. The full sized drop are fine for the bearings of a pre-war Märklin or a Doll stationary engine, but a smaller drop for small fast rotating turbines (like the Karsten Gintchel models I've owned), unless you want your shirt accessorized with a fine mist of oil.
I beleive you would need a bit of patience if you load heavier-weighted steam oil, such the Regner's product, so I stick to lighter oils for my pen.
Dave