The little book that comes with the car with the words "Vehicle Handbook" on it tell you how much oil and what grade they recommend. They don't make it up, so stick with it unless you know what you're doing (and even then you're probably wrong, but at least you won't be complaining to us if you screw up!
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Oil filters usually available from any motor factors.
To change the oil yourself:
Warm engine up with a 10 minute gentle drive (skip this step if you really have no idea what you're doing as burning yourself looking for the right bits is worse than the oil taking ages to drain and not taking all the crap with it).
Locate oil drain plug on the bottom of the engine. NOT the bottom of the gearbox. It will probably be a 14mm AF taper plug, so get your 14mm spanner ready.
Place an old contained under the plug to catch all the oil.
Remove aforementioned plug, and let oil drain into the container.
Whilst that is doing what it does, open the bonnet (hood) and search for the oil filter. It's a round cylinder, probably about two inches tall, on the side of the engine. It'll look a bit like the new one (but might be a different colour).
Use an oil filter strap (or other oil filter tool) remove it by unscrewing it anti-clockwise. When it comes off you will spill some oil, so mop it up quick! The filter may be tight!
Once that's off, clean up the seat of the oil filter area on the engine.
Take new filter, and apply a drop of engine oil onto the seal and spread it over it. This will help the seal.
Screw the oil filter on, being careful not to cross-thread it. Tighten it BY HAND,
NOT WITH A TOOL, but quite tight.
Go back under the car, and put the plug back in if it's finished draining. Don't overtighten it as they are taper threads into cast/pressed sumps, and you will crack it. But a normal size spanner and normal human effort will be about right - just got the judge it really.
Go back to the top and unscrew the oil filler cap (the plastic thing with a picture of an oil can on it on the very top of the engine). Fill it with oil - your book will tell you how much.
Every so often check on the dipstick if you've got the right amount in - too much is worse than a bit too little, so don't overdo it.
When it's at the right level, check all caps, bungs, plugs and stuff are done up. Check for leaks (wiping up old oil is a help here). Then go for a short drive to get the oil up to temperature (10 minutes), and check for leaks again.
You are done! Hurrah!
Should take about 10 - 15 minutes, maybe a bit longer if you've not done it before and/or the oil filter is in a hard to reach place.