During WWII, apparently new M1s were dipped in cosmoline before shipping. This cosmoline was NOT the heavy grease used after later rebuilds, or used on the M1s returned from Greece. The WWII cosmoline was a very dark brown color and was quite sticky. GIs cleaned it off with gasoline, but that was a nasty job.
If the rifles you refer to are covered in real, honest WWII cosmoline, they are likely to be worth more uncleaned than cleaned.