Can I use the same moisturizer in summer and winter?

You can, but you will be more comfortable if you switch. Winter moisturizers tend to be thick creams with heavy occlusives (like shea butter) that feel greasy in humid summer heat. Summer moisturizers are lighter gels or water based lotions with hyaluronic acid that hydrate without feeling heavy. If you have oily or combination skin, the switch is especially worthwhile heavy creams in summer can clog pores and increase breakouts.