What I'm about to post might sound 'weird' but it is fairly accurate nonetheless. Oil viscosity ratings aren't actually specific numbers, as their grading would suggest. What that means is there's a 'range' of viscosity measurement for each number. Perhaps the best way to say it is this - it's possible that one oil is at one end of the allowable range for a 50 oil, and one at the other, so one is much 'thicker' than the other, but both have the exact same rating.

On top of that, the interesting thing is that oil viscosity is not necessarily directly linked to its film strength. On other words, a 'thinner' oil can provide as much protection and keep the moving parts separated by that oil film just as well as some thicker oils. Additionally, a thinner oil will be able to travel through the oil circuitry/bearings quicker, which actually means it can potentially transfer heat away from the bearings better.

So basically you can get the same protection with an oil that is 'thinner' (or if bearing clearances are tight (and by tight I mean that relative to the specified allowange for some other engine make/model, not that they were accidentally put in there and there was insufficient clearance) if they are tight - a thicker oil might actually be detrimental.

I mention all that because BP corse plus happens to be an oil at the very 'thick' end of its classification, so it's thicker than other oils of the same viscosity/index. Generally speaking, it's an option when an engine is worn to the point it's consuming oil, and it can help on that sort of situation. It's _definitely_ about as far as one should go. the 'treacle' or honey like oil additives 'seem' good, but they neglect things like the fact that cooling for valve springs (and lube) relies on oil splash to a certain extent, and cam/lifter foot oiling especially does, and those types of additives might indeed be detrimental there - and actually shorten the engine lifespan.

Far out, what else - diff oil viscosity is not measured with teh same criteria - and what might be a big surprise (it used to be a backyard 'bodgy' option to put about 1:4 of diff oil vs motor oil to an old engine that was using oil) is that whilst it's thicker at room temp, at actual engine operating temps, an 80w/90 diff oil thins out more than 20w/50 engine oil, so it can be counterproductive (not to mention that other additives in diff oil - necessary for the extreme pressures, adn the fact that hypoid gears actually slide across one another, they don't just engage against one another like a gearbox gear does- that make it a bad idea for engine oil)

What next - the bigger difference between the two ratings of oil - i.e. the 20w50 vs say 10w/60 - well the greater the difference between those two, means there are more viscosity index improvers in there. It means it's a thin (and hence is easily pumped so it is there quicker after startup from cold) at room temp. As it heats, tiny 'things' in the oil sort of 'unwind' so they keep the oil thicker at higher temps. These vi improvers can and will get roughed up and basically destroyed with extreme use, or just long term use. So the greater the distance between the numbers, the sooner they'll deteriorate, and you end up with an oil that is far inferior to the condition when you first put it in there.

So where I was going with that, is that obviously go with manufacturer specs for the oil, but if you have a choice of a few oils that fit the bill, generally the one with less gap between the two rating numbers is the go. On top of that, since the vi improvers inevitably degrade, no matter how fantastic the oil is, it's important not to leave it in there too long, no matter if it turns a darker black or not. I'd go as far as to suggest (and this is also because the a-series is a very good design, and isn't actually particularly 'hard' on oil (the hemi 6 in contrast is far harder on it, and some oils are compromised with as little as a day spent driving hard - like a visit to an open day at a racetrack) - anyhoo, what is generally _far more_ important than brand of oil is that it is changed frequently.

You don't need to go beserk and change it every 1000km. Not at all, but using a relatively more affordable oil and changing it every 5000km is in fact generally better at maximising engine life than a more expensive oil every 10,000km. I used to change oil and filter very 5000 on some other cars I've had (minis for example) and tended to (by that change of maintenance alone) get as much as double the life out of the engines as seemed to be the norm.

My honest take on things (and I've currently got a 1200 coupe, 1973 model, still on original engine, and not using oil) is that Id worry about the right grade of oil, and more frequent changes, but not so much any brand loyalty.

There's a _really_ good website that explains oil tech (and if I've made any slipups here, certainly take what you read at that site to the bank) :

bob is the oil guy

If you have the time and curiosity it's well worth a look.

Ironically questions about 'which oil' (specifically brand of oil, but sometimes viscosity) come up in the aus.cars usenet group. Amongst the lunatics and trolls, there are also a few engineers, and engine machinists, with extensive experience with engines, and perhaps more importantly race engines and other highly stressed scenarios, and this is the general consensus of the group (admittedly they could word it a little better than I could).