Why does the B-29 have eight oil needles?
Each of the four engines is represented by two needles, one in each row. The top gauges are for the nose oil; the bottom gauges are for the rear oil.
Nose and rear here refer to two sections (remember that's a twin-row radial engine). The nose case oil is also used by the propeller's governor, feathering mechanism, and reduction gear – those would explain the lower pressure range of the nose section.
During development the nose case was troublesome:

Reduction gear failures started to occur during extended climbs, forcing the addition of external oil lines from the accessory case to the nose case to shore up low nose case oil pressure (enginehistory.org; PDF).



B-29 commander training manual

More on Google Books: United States. Army Air Forces (1950). Technical Data Digest. p. 26.