Blending Synthetic and Regular Oil
I’ll admit that I used to be part of the crowd that believed that synthetic and regular oil could not be mixed. I falsely believed that mixing the two would cause congealing, foaming, and leaks. All of that turned out to be false.
In fact, some major manufacturers produce blended oils known as semi-synthetics. These combination oils work because synthetic oil is based off of regular crude oil and then hydrocracked to eliminate impurities. If you already have synthetic oil outboard motor and regular motor oil on hand, then you can mix your own semi-synthetic. Just be sure to read up on proper procedures before attempting the combination.