Converts botryococcene to mono- and dimethyl derivatives, but not to tri- and tetramethylated products. Unable to methylate cycloartenol, zymosterol or lanosterol, but can also use squalene as substrate. Methylates both C-3 and C22 positions, but only C-3 position in monomethylated squalenes. In contrast, monomethylated botryococcene occured mainly at the C-20 position yielding showacene, but also at the C-3 position yielding isoshowacene. C30 botryococcene + 2 S-adenosyl-L-methionine = 3,20-dimethyl-1,2,21,22-tetradehydro-2,3,20,21-tetrahydrobotryococcene + 2 H(+) + 2 S-adenosyl-L-homocysteine Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.