Old school rust proofing was 50/50 old engine oil and paraffin. Worked well if done regularly. 5 W 30 5 W 40 is only recommended were temps go down to -30 degrees. In our climate 10 w 30 or 10 W 40 is what to go for which is in the -10 to +30 degrees range. On an engine that has done some miles i would say 40 is better than 30. Handles the heat better when tolerances maybe growing due to slight wear. And as said, a can of STP does no harm at all on each oil change. Gives a little bit of help to todays low Zinc oils.