I usually work with ink and watercolor on mylar (paper used by architects). I tried to use oil paint on mylar and the effect is real nice. Do I have to protect the finished painting with a varnish or a fixative, is it crucial?
By Mylar I assume you mean drafting film (usually a (semi) transparent type of BoPET (biaxially-oriented polyethylene terephthalate)), rather than drafting paper (or vellum).
Drafting film generally is coated, although the uncoated version is preferred for archival purposes.
Drafting film, especially the more common coated version, does not absorb the oil in oil paints. Along with the high flexibility of the sheets, this presents a great risk to both the drying process and adhesive support of oil paints.
This means that, apart from the protection it offers the paint film itself, varnishing or fixating alone will do very little to preserve the painting on mylar.
Moreover, covering the work with a coating can effectually enclose the complete painting, making it airtight, and, unless the paint is truly cured, this might cause increased deterioration.
There are a few things you can do to increase their integrity:
- Using very fine sandpaper, slightly rough up the front surface (both surfaces, in case you want to mount the sheet);
- Ground it with a little (diluted) gesso;
- Use only thin layers of (diluted) oil paint;
- Let it dry sufficiently in a stable environment;
- Mount it to a rigid surface;
- Frame it behind (or between) glass.
When mounting, you have to make sure no blistering occurs. You can use acrylic gel to cover the entire surface, smooth it out by hand, and let it rest for a few hours, while applying equal pressure.
You can also try double-sided adhesive sheets, working very carefully from one side of the sheet to the opposite.
More info on mounting in the comments here.
A really nice overview of working with paints on Mylar can be found here.