Mineral and synthetic oil mix readily. Old generation vegetable (castor) oil does not mix with them, but you won't generally find that these days anyway. Obviously synthetic (or semi) is best for lubrication, particularly in hot conditions but if you need to top it up you can use mineral - better than letting it get low. Try and keep to recommended oil and filter change intervals (oil does get contaminated and degraded) and *if possible* use semi or full synthetic oil. The preferred grade for hot weather would be xW-40 or xW-50.
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