Oil type

I need to know the correct oil weight for 64 Dart. It has a slant 6 in her, sat up for 20 yrs,but before that,it was overhauled. I have been driving it now for 3 yrs,am using valvoline 10W40 regular oil. I will not use synthetic, so I wanna make sure the right weight is correct and also should I start adding zinc additive to it. She does not burn any oil and runs great. Thanks in advance for any info. Todd