What year is your car? If I remember correctly only the 87 models had the ETACS unit that drained the battery. The 88-89 models were corrected by Mitsubishi.
The owner's manual says you can use 20W-20, 20W-40 & 10W-30 in 32 degrees or above. 5W30 in 60 degrees or lower. I've personally used Castrol GTX 10W-40 in mine for years without issues.