“It is like precious oil poured on the head, running down on the beard, running down on Aaron’s beard, down upon the collar of his robes.” (Psalms 133:2 NIV)
The idea of someone pouring oil on my head and clothes is not something that fills me with delight.  But I would be missing matters of great significance if I were of the generation of the Psalmist. Oil symbolised things that were of spiritual importance.
For example, oil marked someone out as a priest,
“Take the anointing oil and anoint him by pouring it on his head.” (Exodus 29:7 NIV)
Christians today are ‘priests’ too (1 Peter 2:9). When we see each other as God’s anointed we will treat each other differently and unity becomes possible.
Oil consecrated things and made them acceptable to God,
“Make these into a sacred anointing oil, a fragrant blend, the work of a perfumer. It will be the sacred anointing oil. Then use it to anoint the Tent of Meeting, the ark of the Testimony, the table and all its articles, the lampstand and its accessories, the altar of incense, the altar of burnt offering and all its utensils, and the basin with its stand. You shall consecrate them so they will be most holy, and whatever touches them will be holy.” (Ex 30:25–29)
When we remember that our fellow-Christians have been consecrated by the blood of Jesus it helps us to be patient and loving – creating unity (Heb 10:22, 12:24).
Oil indicated God’s presence, and was a symbol of the Spirit, bringing warmth, perfume and peace. Banquet guests in Old Testament times were given fine oils to anoint their foreheads. This made faces shine and, being perfumed, made the room smell pleasant – very important in the hot climate of the region.
When we recognise the Holy Spirit lives in one another it helps us to be welcoming even when we have differences, and then unity grows.
The oil for a priest was  poured on the head, but also  sprinkled on the robes,
“And take some of the blood on the altar and some of the anointing oil and sprinkle it on Aaron and his garments and on his sons and their garments. Then he and his sons and their garments will be consecrated.” (Exodus 29:21 NIV)
As the oil is poured on the head it spreads to the clothes.  We are to be together as the people of God in that same sense – different but integrated.  The joy of one disciple spreads to another, the grief of one Christian spreads to the other.  We share in everything. This sharing unifies us all the more.
“Share with God’s people who are in need. Practice hospitality. Bless those who persecute you; bless and do not curse. Rejoice with those who rejoice; mourn with those who mourn. Live in harmony with one another. Do not be proud, but be willing to associate with people of low position. Do not be conceited. Do not repay anyone evil for evil. Be careful to do what is right in the eyes of everybody. If it is possible, as far as it depends on you, live at peace with everyone.” (Rom 12:13–18)
Let’s spread the love and the unity will follow.