If its cold outside and the thicker oil is colder it will take time to warm up and thin out enough to get around all the parts. Does it still do this even after warmed up? If not then the clutch may need to be adjusted a little, there may not be enough slack in the cable and it has a slight tension and is letting the clutch engage, if thats not it then it may need to be replaced. May change the oil if you haven't yet, having gas in the oil of course is a bad thing it will break down and thin out the oil and is not good on the parts.