Hi Tony99, On the oil type to use you can go genuine oil which will definitely do the job, after market oils are good too - the grade of oil should be listed in the owners handbook if the car came with that. Other than that most of the retailers will have a look up chart where you can confirm the type of oil brand and grade is best. Bear in mind that you want to make sure you do see a listing for your specific model - assume the DC5R has a LSD factory fitted. I've used Castrol VMX 80 successfully in a LSD equiped fwd honda gearbox.
The metal wearing parts (very very fine particle sizes) will drain out as you remove the old oil but make sure you give the drain plug a really good clean as these are magnetic at times and will hold small bits of debris to ensure that they don't cause premature wear in the 'box. Depending on the breather location on the gearbox you shouldn't really have any sediment/gunk in there unless the box has been submerged for some strange reason....
With ambient temperature this is normally a concern at the extreme ends of the range - eg very negative Celsius and very high celsius, normal operating temperatures ranging from -5 to +45 you'll find the standard stuff works well. If you're operating at the top end of the temperature range consistently then more frequent oil changes would be a good preventative idea. In very extreme conditions you'd look to fit an oil cooler however these are more inclined to be needed in automatics under heavy load like towing applications.