'...may have had a problem with rim cracks, is this true? '
yes, sometimes. if you notice cracks, change the rims against the ones of an '85 model (55W). they have the new type of rims.
'Also, which oil is the best to use...'
you should use a good quality MOTORCYCLE oil because in your bike the oil has to do more jobs than in a car engine (clutch, gearbox).
the brand is not so important, but the API-specification is. SE is low quality, SG is high quality oil.
if you ride more in high temperature conditions a xW50 should be prefered, else a 10Wx.
i don't want to start a discussion here about mineral or synthetic oil. in my opinion mineral oil is good enough if you use your bike 'normal'.
but whatever type and quality of oil you choose, the most important of all is:
-carefully warm up the engine, ride at least 20 km until you use higher revs (<4500)
-perform regular oil change every 5-6000 km
-keep the oil level always at maximum
and a very useful modification would be to mount an oil thermostat (if the bike is equipped with an oil cooler). this helps to warm up the engine more quickly.