What oil do I put in this air compressor?

[Tom300M]

A relative has this compressor in his independant shop. It is from the 70's I think and sure looks like from the sight glass that it could use some type of oil, but I'm a little wary of just putting in some SL rated 10W-30. Any ideas on what I should put in?

 

[bruce381]

for a old recip use a good 30wt straight grade.
bruce

 

[JHZR2]

ND30

 

[Tom300M]

Thanks guys

 

[wileyE]

My old perkins compressor (probably 60's or 70's vintage) calls for type f ATF. Looks red in the site tube, never had to add any.