I use half olive oil half coconut oil. It does take on a distinctive coconut oil flavor though, but I like it that way. The coconut oil also gives it a margarine like consistency, not runny like home made mayo sometimes is.
My recipe is
1 egg yolk
1/4 c each oo and co
1/2 tsp dry mustard
1 TBLSP lemon juice
salt to taste
I kept having bad luck with electric blenders with separation. So I finally stopped being lazy and do it by hand with a whisk. It actually seems easier that way and keeps a better consistency.
I blend the mustard into the egg yolk first, then add the oil about a tsp at a time. After all the oil is emulsified I add the lemon juice and salt.