Got this info` from Jamies site.

http://www.dreamgate.ne.jp/NSR/

MC18 1988 model 2J and 4J 128mm
MC18 1989 model R5K 136mm
MC18 SP 1989 model R6K Terra colors 150mm
These are oil levels, which are measured from the top of the fork leg at full compression, with no spring. The Honda service manual recommends 10W oil.

MC18 1988 model 2J and 4J 429cc
MC18 1989 model R5K 435cc
MC18 SP 1989 model R6K Terra colors 374cc