If you are only topping up in between services,say less than half a ltr in a year using 502 this surely would not be enough to dilute or effect the other 4.5 ltrs of oil in the engine ?
Well, to slightly pedantic first, if you need to add half a litre, your sump volume will have reduced to just four litres - which means somthing like 12% loss of oil.

But to answer the comment, using upto a maximum of half a litre of non-approved oil is perfectly acceptable for top ups between services. However, from experience, you will need a lot more than half a litre for top up - as virtually all VW Group engines are known to have quite a high oil consumption for the first few thousand miles. Generally, they take anywhere between 8k and 13-14k miles before they stop drinking oil. But this is highly dependent on how they are initially run in.
, i still can`t get my head around the vw golf gti owners handboook saying to use 502 on non longlife service and this being incorrect 
Because Volkswagen, just like all manufacturers, are constantly evolving and improving.
But to try and clarify further - if you are on T&D servicing, and you, the driver/owner need to top up (or even want to do your own oil change), then 502.00 is perfectly acceptable. However, a 'better' oil is the 504.00 (for a whole range of very technical reasons).
But what peeps seem to be getting confused with is what Volkswagen WORKSHOPS are allowed to use, and what Volkswagen RETAILERS are allowed to sell you. VW workshops are NOT allowed to use anything other than 504 oils, and VW retailers are not allowed to 'sell over the counter' non 504 oils. But if you want to use an independent workshop, you
can use 502 oils - but as I have repeatedly stated, 504 oils are considerably better - and are much more preferable and suitable for the turbocharged GTI engine.

Has anyone actually read this thread:
http://www.golfgtiforum.co.uk/index.php?topic=65779 
Can anyone say what standard oil all gti mk6`s are filled with when they leave the production line and if it was 502 can you top it up with 504
Jesus - can't anyone be arsed to search the forum. It has been long established that ALL Volkswagen Group engines, for ALL cars, irrespective of target market - are factory filled with:
So just in case you don't get it - that is: