Bring milk, oil, butter, and sage to a boil in a small saucepan. Remove from heat and set aside. Cover potatoes and garlic with 2 inches cold water in a large pot. Add 1 tablespoon salt and bring to a boil. Reduce heat and simmer until potatoes are tender when pierced with a knife, about 13 minutes.
Drain potatoes and return to pot. Pour milk mixture through a fine sieve over potatoes; discard solids. Mash potato mixture with a potato masher to combine. Season with salt. Mashed potatoes can be made 3 days ahead and stored in refrigerator. Reheat in a bowl set over a pot of simmering water, thinning with more milk if necessary.