Ok I done a bit of searching and not found much already listed - maybe my bad searching but I though I'd post my own experience with this.
My grip came lose on my GRDII as they do - heat and all. After much searching I finally settled on 3M's thinnest VHB tape and after cleaning / removing all the adhesive from the previous tape I can confirm that this works well and in hot/humid environments. It's not been a year yet but I think it will be at least as good as what Ricoh supplied originally.
I didn't buy a roll - instead a emailed #m adhesive and they sent me a sample roll which is plenty enough for the Ricoh. So for anyone out there who needs this this may be your solution.