Any oils/agents that help desensitise penis skin so ejaculation is considerably delayed/last longer?

Not a good idea: Permanent desensitization of the skin is not a good idea. You may use condoms that contain a local anesthetic which may help in lasting longer. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.