Battery...only add acid if you spill some out.  If it has places to check the electrolyte level, add distilled water up to the minimum mark, fully charge, add to the max mark.  Charge these little batteries at a max of 2 amps, and trickle charge if needed for long term storage with an automatic charger in the 0.75 amp to 1.33 amp range (or a 1-amp manual charger for one day a month).

Oil...Diesel engine oil is fine, but for very hot summer climates I prefer a 20W-50 motorcycle oil or somethingW-50 synthetic motorcycle engine oil, or 20W-50 car oil IF the data sheet says it's OK for wet clutch motorcycles.  Never, ever use Castrol Syntec...the only oil my mechanic can count on to make the clutch slip.

If there is rust in the gas tank, motorcycle shops sell 2-part kits with a cleaner and a coating.  Be sure to clean any rust flakes from the screen on the petcock.

Be sure the tires are aired up to at least the minimum shown on the sticker on the bike.  The date the tires were made is the last four digits of the DOT number...the week & year of manufacture.  If they're old (5 years or more) or showing signs of cracking, replace the tires and tubes.  A new rear tube with a 90° bend in the valve stem makes it easier to check the air.

Hiflofiltro makes very good oil & air filters at about 1/3 to 1/2 the Suzuki price...buy at any non-Suzuki shop--give them time for a special order for you.  I've read that Hiflofiltro makes the motorcycle oil filters for K&N.