I know witch ones they prefer, but I think it usually depends on what the contractor brings out.

This has been my personal experience on pipeline :  5p+ works great for bead and hot pass ( as long as it has been KEPT DRY, If it gets damp its no good for anything ). The 5p (red) is good too, just a little sticky, but not affected by moisture as much. Fill and cap are usually 7010 HYP or 8010 ( 70+ ). The 5p+ is not very good for the cap because it will leave pinholes when welding downhand. The 5p is o.k. for capping if the contractor or oil co. will let you use it. Most pipeline contractors around here will supply the welders with the appropriate rods specified by their weld procedure or by the owner of the project .  - WS