Engine oil is for engines, gear oil is for transmissions.The one size fits all doesn't cut it in real life. That part number you posted is the Formula+ from HD and you can use it in the primary, I would use a gear oil because of the differences in the composition between the two in the trans like Spectro platinum, redline shockproof or Mobil 1 75/90 or 75/140 but all gear lubes and in the trans.