Normally I don’t use inexpensive pens, but lately I have found three inexpensive pens I now use for work and when I am travelling due to the good ink they contain. They are the Pilot G-2, Pilot G-TEC-C4, and now the uni-ball 207.
My current favorite, the uni-ball 207, uses an ink that contains color pigments which are absorbed into the paper fibers. The ink is in effect trapped on the paper fibres and can’t be washed off, as some forgers do to alter cheques. Refills are available. Uni-ball 207 is sold worldwide in stationery and office supply stores and other outlets. Mike Shea did some interesting tests of the ink in the uni-ball 207 and the G-2 and three other inks. The G-2 survived water but not soap and bleach. The uni-ball ink survived all the tests.
The Pilot G-Tec-C4 or the G-TEC-C writes with a very fine line. I use it for corrections and margin notes. They are so thin they write like mechanical pencils. The G-TEC-C seems to have a more durable ink, but it is hard to find in North America.
If your handwritten records have to survive intact for a long time, then you have to carefully consider the ink used to produce them. It seems I’ll be using the uni-ball 207 a lot more from now on.