Hi There!
Halfords Vintage 20w50 is what I use.
Steps:
Run the engine until its warm, above luke warm but below finger burning warm!
Open the oil filler cap, open the oil bung and be sharp to have a catch can below and leave it to drain.
Note the position of the timing mark on the oil flinger cover, rock the engine in gear until the mark is at 12 o clock, then begin to loosen the bolts on this.
Replace the bung in the sump, nice and tight but don't over tighten. Make sure all threads line up, if you ring the sump your in a bit of bother.
Move your drain can to under the flinger, take the nuts out of it and pop it open, there will be another gulp of oil that'll burst out.
Remove the o ring and soak the front cover in kerosene or brake cleaner. Scrape any gunk out of it with a screwdriver or stanley, same with the back part, I normally do this in situ, you can take it off but that's a bit more effort. Can fine clean it with an old toothbrush.
With everything clean, pop on a new o ring (part number 987112) and reassemble remembering to put the timing notch at 12 o clock.
Measure out 2.6 l I believe (assuming your sump is standard and not an after market aluminium one) and fill nice and slowly, remember to dip it throughout filling to check your level.
Replace oil filler cap and you are good to go.
I have never seen a seal on the sump nut of an 850, 600 or on a 500 with the 650 engine. mine could well be missing on the three engines I have, snug fit seems to do the trick.
All of this is good to do with the car in gear as it'll stop the engine turning over as you open and close the nuts on the filter.
I think that is it, all very easy to do with a good socket set, flat head, Stanley, brakeclean/kerosene, old toothbrush, new o ring, container to catch oil, container to soak flinger front plate and a measuring jug.
All simple stuff but it's good your asking and learning!
Gav
