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Security DVRs will use “strong” compression. This compression enables longer recording times but it also results in a loss of SNR (lower quality). DVRs are better than nothing but not as good as PCs recording uncompressed, DV camcorders, or event old VCRs.
A lot of the vast file space-saving in security DVRs is done by reducing the frequency of capture, not by compression, and that is also why they can be useful for long duration events like eclipsing binary stars. If you want good time resolution they can still capture at the native camera rate of 30 frames/sec.
And regarding compression, these old Sanyo DVRs use M-JPEG, which is essentially just a long string of JPEG images, and you can select how much or little compression you want (there are five levels of image quality). M-JPEG is similar to the compression used in Digital Video — which is commonly used for recording occultations — and both of these are less aggressive than the H.264 / MPEG-4 algorithms used in newer hardware (M-JPEG file sizes are several times larger than with H.264). As with DV, M-JPEG compression is entirely intra-frame, i.e. it doesn’t use predictive information from adjacent frames, so it is well suited for occultation s because it doesn’t “blur” an event in the time domain.

I have only evaluated a few recent DVRs which were bought in the last few years. In this testing I have seen some DVRs which drop fields/frames as a means of “time compression”. But they all continue to use lossy compression for the field/frame data as well. I not seen any DVR which has an option to store data with lossless compression. And current models seem to use H.264. Your older DVR with m-jpeg compression sounds like a reasonable option. But I hesitate to recommend DVRs for recording because I do not know of any current generation DVR which doesn’t use strong compression and field dropping.