Synthetic or not?

On all the bikes and cars i've owned for tens of thousands of miles ive used Castrol GTX semi synthetic car oil with no issues with clutch slip or oil loss in the bikes (car has always used some though).
GTX full synthetic is on offer for £16 for 4 litres, is there any harm at all in switching to it? I've heard rumours not to on higher mileage vehicles as the part synthetic can help block areas which would otherwise begin to leak, its also apparently harsher on seals.
Bike is on 13k, car is on 75k.

I have always used Car Oil for Cars and Bike oil for Bikes. Semi for older & higher mileage & fully synthetic for newer or high performance engines. My two Suzuki's are using Shell AX7 Semi (http://www.opieoils.co.uk/p-73843-shell-advance-ax7-10w-40-four-stroke-engine-oil.aspx) and the BMW is using Shell Advance Ultra Performance Synthetic Bike Engine Oil (http://www.opieoils.co.uk/p-69341-shell-advance-ultra-4t-10w-40-performance-synthetic-bike-engine-oil.aspx).
As far as I am aware semi & fully synthetic will mix without any issues, but I personally would not put fully synthetic in the VL and I would definitely would not put car oil in it, but that's me. Car oil has friction additives in it and they are also not designed for the 'shear' forces within a bike engine, but in saying that the VL engine does not rev to 14k so it probably is not an issue.

Personally I only use bike oil in my bikes and would never use car oil. A mate of mine who has a Honda vt 750 Shadow used car oil and the clutch failed in less than 500 miles, repair shop insisted it was the direct result of using car oil.
This is only my personal opinion, everyone has to make thier own decision as to what they think is the best and safest oil to use.

stingray wrote:

Personally I only use bike oil in my bikes and would never use car oil. A mate of mine who has a Honda vt 750 Shadow used car oil and the clutch failed in less than 500 miles, repair shop insisted it was the direct result of using car oil.
This is only my personal opinion, everyone has to make thier own decision as to what they think is the best and safest oil to use.

Depends on oil type, any oil branded as energy saving apparently have friction modifiers or something which causes the clutch to slip, but an oil change should eventually stop the slipping once the old oil has worn off the friction plates. Or the clutch was on its last legs anyway and its just coincidence.
Regardless I ran my GZ125, Virago 535, VL800 and now C800 all on GTX semi synth and never had a clutch go bad, or leaks.
But to be on the safe side I think ill stick with GTX semi synth and avoid fully synth since I dont know what oil type the engine was broken in with

Bought this in the end

http://www.ebay.co.uk/itm/371317089949?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

The old Castrol GTX semi which came out looked fine to be honest, only covered 700 miles or so last year then it sat all winter.
But 5 litres for that price is reasonable since the bike only took 3400ml.

I used to use that in mine, well up to the job