well good 15" tires will cost around RM180 to RM300, while 16"/17" tires will be around RM230 to RM400... where it is a wide RM50/tire gap there if you don mind u can change to 16" or 17" rims... the tires of 16"/17" is not that big difference though... 205/50 R16 or 205/45 R17 is good for you... if u wanna stick to 15" try out 195/55 R16...
of course 15" give you more comfort while not bad on handling, but 16"/17" will be better in handling but suffer on comfort... ur choice
not really la... they use Semi Syn... Mineral engine oil is a lil thicker 20W50 where protection to the engine is more... but ours is newer generation of cars so 10W40 engine oil is good enough... but it is slightly more expensive lo... and it last a lil more...
for 20W50 normally ppl advice around 5000km change, 10W40 is 7000km while the Full Syn 5W40 is around 10k km... so it is ur choice... cos better interval change i think full syn is better coz it save u all the fuzz of changing oil after 5k km... and the time it need... you can roll back from Full Syn or Semi Syn to 20W50 (Mineral) engine oil anytime but not the opposite direction... it is still owner's choice... i still use 10W40 for my SEG 5AFE engine...