I changed to fully synthetic oil recently. Over the next few thousand km the engine began to smoke at start up whether parked on the side or centre stand. I suspect this was the valve guide oil seals. The engine has also smelled fumey. The rear seal has also given way and oil has been leaking through the weep hole in the bell housing and my clutch has been slipping. My engine had no leaks before this and has only done 150,000km...it never used any oil other than what I would take out.
I changed the oil for fresh synthetic and used an engine stop leak additive. This is meant to rejuvenate the rubber seals. I haven't gone far since... the weep hole is still leaking but I might have noticed a slight reduction in smoke at start up.
Can anyone suggest a longer term plan?