I have recently bought a 2003 Rancher an I don't have a service manual or the owners manual. My question is any advise on what oil and oil filter to use? The ATV runs very strong but seem to have a hard time going thur the gears the 1st time when riding. No problems after that hoping the oil change might help ...any advise would be welcome thanks !

I use rottela t 5w40 synthetic in my 07 rancher 420 without any problems, I also buy the fram filters and have yet to have any issues with using them.
I buy both from my local superwalmart easy cheap and usually open when I need them to be. You can also use any 10w40 designed for motorcycles with wet clutches, just dont buy automotive oil that you would use in your car or truck.

I agree, do not use regular car oil. Look for an oil with the JASO MA or MA2 spec on the label. Not only must the oil lubricate the engine but it also serves as a transmission lubricant. That's a component of the JASO MA/MA2 spec. Many auto/truck oils may not be suitable in those conditions. While these oils may appear to be ok in the short run, long term use can lead to gear pitting in the trans and clutch problems. Also some of these automotive oil have anti-ware (sulfated ash) additives. Sulfated ash can cause pre-ignition if the oil is present in the combustion chamber. It can also contribute to deposits above the piston rings and subsequent valve leakage.

Honda specifically states in all their manual to no use any oil that has the words on the label "Energy Conserving". Those oils are for fuel economy and have less friction which works against the transmission/clutch system which needs a certain amount of friction to function.

As for oil filters, I prefer Wix. The one for your model is 24944. The Wix is also available in the NAPA Gold 4944.

I have always used AMSOIL AFF 0W-40 in my Ranchers, the 01 and the 05 with good results.

I use Bel-Ray EXL in my rancher with a K&N filter and it has ran great with it. 10,000 miles and it still shifts like the day i bought it.