Recommended engine oil for AC G153 engine

Hello,

Any good SM rated oil in 10w-30 5w-30, or even 0w-30 should work very well, especially in cold weather.  (There is no such thing as a winter rating, ie at 32 degrees{10w/5w/0W} that is too thin, even 0w-30 oil is to thick at freezing temps but is the best we got)  At 210F/100C all 30 weight oils have the same protection.  The ratings/recommended oils from that time period when your engine was built are nothing like what is around today and are not comparable.  Multi grades were non existent at the time.      The sage advice is 10 psi of pressure when at full operating temp for every 1000 rpms.  The engine is probably governed somewhere around 2000 rpm.  You should use the thinest oil that will maintain 20-25 psi at operating temp.  Thicker 40w or 45w oil is NOT better if you can maintain oil pressure with a lower viscosity oil.  Thicker oil only helps in larger clearances do to wear. 

15W40 is fine at 90 and above.  Diesel oil can also be had in 10W30 , I use Mobil Delvac 10/30 in my powerstroke and Kubota 4150 year round. they start much easier in the winter and in my climate we don't see much above 85 ever. Even 10/30 gets thick at 20 below which is not uncommon here in winter.