Yeah, you'll be fine. Switching from ordinary mineral oil to fully synthetic can cause problems, as synthetics can cause oil seals to shrink; mineral oils cause them to swell slightly to keep the oil in and they wear over time, so any shrinkage causes a leak. Crankshaft oil seals were the favourite for this one. Semi-synthetics are just about neutral in terms of seal swell characteristics, so you should get away with it. If we are on about 2-stroke, don't worry about it at all. I worked in the lubricants industry when synthetic motor oils first came out and we used to take loads of calls from people that had spent £20+ on Mobil 1 or the like, only to find that it was leaking out of the engine. Our advice was to go down to the supermarket and buy the cheapest, nastiest £2.99 per gallon own brand motor oil that they could find and replace the synthetic oil with that, to swell the oil seals up again and the leak would slow down and eventually stop. Needless to say some people were taken aback by our answer but it did work!
