I know I'm late to the party but just to add to the answer from R18:
Sadly there are quite a few of "older" movies on R18 that are only provided in SD but actually have an HD release. You can check for example on duga.jp if there is a better version available. Most of the time (but not always) duga.jp has the highest quality release. But knowing there is a better quality available can be a curse if you are not able to get your hands on it. :/