A couple of years back I had the original 352 engine replaced with a 390, in my 1965 Galaxie. The mechanic who did the swap used 20W50 oil and I have continued to do the same whenever I got an oil change. When I go back to the copy of the service manual it shows that the recommended oil is 10W30 or if consistent temp below -10F use 5W20. Does anyone have any suggestions as to whether I should continue to go with the 20W50 or go with the service manual recommended 10W30? Also, any recommendations as to the benefits of synthetic vs non-synthetic oil?