You cant always just switch to a higher weight oil. You should always use what is recommended. This is what the engineers designed the motor to run on. That weight oil is what supports all your bearings. If its too thick, it wont get in between the bearings and races and will cause damage. The older cars were able to run this heavier oil because the tolerances were not as tight.
Dan

92 Rodeo, 3.1 TB crank, custom bumpstop spacers, DOR shackles, Flipped ball joints, D44 Rear, 4.56s and new magnaflow cat and dynomax ultraflow muffler Since been replaced by a 2 door Explorer on 31's shackles, cranked torsion bars and full exhaust