Heat water to boiling in a pan, add potatoes. Reduce heat, cover and simmer for 5 minutes. Add salt, oil and carrots. Simmer 8 more minutes. Add peas, corn, and zucchini. Simmer 5 more minutes, or until veggie's are crisp-tender.
Mix together the milk and flour. Add to pot, and cook, stirring constantly until soup is thick.