Funny you should bring this up since i'm rewriting the dro stuff a bit, although i'm sticking to the msec values. Should produce smaller files for opl3 data especially since there's no index switching anymore.
If you can hear sub millisecond differences then kuddo's to you. These are dosbox's internal milliseconds on which the whole timing is based not the operating system milliseconds, so it should be pretty accurate.

ahaaaaa 🙄 and no any chance to store the value of timer?

Well don't really see the need for logging the timer, say the game doesn't even modify the pit timer so that's still at 18.2hz, but it uses the vga retrace timer for adlib timing. Then you'd basically log big chunks of data on 18.2 intervals and the timing would be screwed. And i don't really think your avarage old computer has any trouble writing out a little bit of adlib data and then waiting for when to write the next when all it's doing is playing the dro file.

why not use the "raw" adlib format? 😀

"raw" doesn't support opl3 or dual opl2 and the timing scheme it uses is basically what deeped said with using the pit timer, which isn't exactly perfect and kinda annoying when you can just use milliseconds.