Follow JPV's suggestion and look for cracks around the bore where the cap/key fits into the track frame before thinking about changing the oil. Try removing the 4-bolt cover just above and behind #31. This is where the track tensioner is located.
If you can suck some oil out from that hole using a hose you can lower the level enough so that no oil drains out when the cap is removed. That oil is generally clean and does not degrade too much, so if you wanted to re-use it all it needs is to take appropriate precautions when sucking it out. Especially true if you find a crack when the cap is removed.
On your maintenance I would only use Cat filters and I think Nige would agree,
Personally I have confidence in Donaldson filters in addition to OEM. Fleetguard I would definitely not recommend based on experience. Other brands apart from those two - dunno.
Lubricants - anything branded from a major oil company that meets the relevant Cat spec for the compartment.
This means nothing where the words "universal, tractor, or farm" appear anywhere either in the name of the product or of the store selling it.
Often Cat dealers do promotions on Cat oil & filters that makes them very competitive price-wise with other sources. I would always suggest to check dealer prices first to use as a comparison with other sources when buying oil & filters.