I have not had a problem buying filters, mine is April 08 build, i give them my rego papers and the parts guy goes and gets the filter. never had a problem now ready for my 45,000k service will get one this week.
all i could think of when i saw the earlier filter is larger , they might be trying to raise oil pressure, if you put a smaller filter that can raise oil pressure, BUT it will need changine slighty more as can be slightly more restrictive with less volume . but would not be that dramatic to worry about.
mobile oil is american so well marketed, in USA they think it's cheap, price Redline oil that cost me $150 for 4L's but i don't use it in the i30 i use same as Daz and change it every 7,000k, the service guy i use wants me to switch to ELF, i think ELF & Redline are the only two syntetic oil's i have personally seen work better as i have inspected motors that run on them in extream condition's.
i've never used Mobile so could not say.
but my friend that builds a lot of different motors and dyno's, also pulls motors down and rebuilds for special aplication's has been saying good things about Castrol Oil and he normaly does not talk that much about oil's