An angel is explaining to Zechariah the work of the Spirit. God's Spirit, moving and producing works (or results, visible signs of God's inspiration and involvement in His servants' activities), is shown as flowing into the church. Here is a short quotation from Keil & Delitzsch.
Oil ... is used in the Scriptures as a symbol of the Spirit of God [notice this qualification], not in its transcendent essence. . . .
The Holy Spirit often appears in the Bible, not in is pure form (the essence of God's mind and power), not in its raw form, we could say. Here it appears as oil. Continuing:
. . . not in its transcendent essence, but so far as it works in the world, and is indwelling in the church.
The oil that is flowing through these pipes is not the raw Spirit of God, but it is His Spirit seen in its works, that is, in its manifestations. We are not seeing, necessarily, God's Spirit as God's Spirit. We are seeing God's Spirit as it manifests itself primarily in spoken and written words, but also in things like miracles, healings, casting out of demons, acts of faith, good works, etc.
Richard T. Ritenbaugh
The Two Witnesses (Part 4)