Turning on H.265 for higher quality video at lower bandwidth is possible through the advanced settings section. If the host does not support H.265, we'll revert to H.264. If your Windows guest computer does not support H.265, we'll revert to H.264. If the host supports H.265 and your macOS High Sierra computer only has software decoding available, we'll switch into software mode, which is going to be slow - you probably don't want this unless you have a really new Mac.
This is currently only supported on Mac and Windows computers.
Turn on H.265 by writing this value in your advanced settings: