BILBAO, June 23 (Reuters) - German premium carmaker BMW is sticking to a "single digit" percent 2010 sales growth forecast, despite outperforming in the first five months, as tougher comparatives and global fiscal changes kick in. "We're still cautious on the basis that the first five months were 14-15 percent growth, but of course the second six months are year-over-year comparisons when things were starting to recover during 2009," head of sales and marketing Ian Robertson told Reuters on