Originally Posted By: UltrafanUK
If you compare oils it's best to compare ones in the same API or Acea groups, otherwise there will be differences.
For those readers that don't look at VOA results, or are tempted to use non major brand oils, I would bear in mind that the worst performance supermarket brand oils available are often 20w50's.
Thinking that thicker oils reduce engine wear is just not correct, as you slow down the oil flow around the top end it results in hot spots, shorter HG life and although it might be good for the main bearings, higher upper cylinder wear rates. Most petrol engines are design spec for Xw30's and most disels for Xw40's and you should only use heavier oils if you have a good reason, like badly worn main bearings.
Moving up one grade seems to be OK in most cases IF the oil flow around the top end is good (Use an idle flush etc), BUT moving up 2 grades is defintely not.
Stuck rings have several different causes and in reality are very rare these days. Detonation issues from the use of bad fuel, bad injectors and too long an OCI or using the wrong oil can all cause rings to stick, although corrosion from long periods of storage is probably the main cause.
There have been a few design issues with higher ring pressures and the use of 20 grades in some engines causing the oil control rings to stick.
If you do have an engine prone to the oil scraper rings causing trouble, don't use thick oil as it does not clean as well as the OEM spec oil will, assuming it has the same amount of detergents. The best thing to do is use a good cleaner oil like Mobil 1 0w40, Amsoil or best of all Shell Ultra or the Penn equivalent AND use a major brand idle flush just before changing the oil & filter. If the compression rings are sticking, make sure the timing is correct, use good fuel and make sure the injetors are gum free by using a major brand fuel additive.
Piston soaks do work but are messy and no good for a diesel. If the block is worn they can result in difficult starting issues in winter, due to the resulting compression loss if Carbon deposits were helping the rings to seal.
So many assertions, so many points to refute! From the top then...
Supermarket 20W50s are the worst performers. Really? Supermarkets don't actually make their own oil, they buy it all in either from a major (Tesco usually have Castrol oils on their shelves) or an independent oil blender (for example Morris Lubricants) who put oil in Tesco own-brand cans. The independents don't do oil tech themselves. They go to the big AddCo's like Lubrizol and buy DI, VII & PPD which they blend with base oil, can and distribute. The 'recipe' oil blends they make are fully supported by exactly the same raft of engine & rig tests that support the equivalent major's oil. Given that engine test programs are so horrendously expensive and are, in the main, developed by the AddCo's, it's not unusual to see a major and an independent using EXACTLY THE SAME oil system. And because of the way matrix oil development works, the 20W50 will if anything be over-formulated because it contains the same amount if DI as say the much more severe 10W40 grade. Might the 20W50 be cheapest oil on the shelf and have the least luggage labels (maybe it just says SG/CD) on the side of the can? Well yes but things aren't necessarily what they seem. Classic marketing drives everything in the direction of offering a 'meets needs'/better/best cascade that shifts the price upwards. 20W50 is the 'meets needs' grade. If the 20W50 in reality did SM/CF/A3/B4/MB/VW/etc why would you put that on the can when you could just put SG/CD to cater for those people who always buy the cheapest stuff? So in short, supermarket 20W50's are in my experience good oils.
Next, thicker oils do not reduce wear...
If that is correct, then you've just turned one if the fundamental principles of oil formulation on it's head. The API's Sequence IVA and ACEA's TU3MS wear tests function in tandem with their associated Viscosity Grade Read-across Tables which have evolved over many years to reflect an established reality. These tables are predicated on the basis that wear improves as oils get thicker and as base oil viscosity increases. If there's a legitimate 'yes but' challenge to this, then there are a lot of oils out there won't be fit for purpose and I don't believe that's the case.
As regards 'thick oils slowing down flow to the top end', this just doesn't sound right to me. First off, your oil pump is a constant volumetric pump designed to shift a fixed volume of oil for a given engine speed. Its throughput isn't affected by oil viscosity like a centrifugal pump's would be. And having said that, if you go back to the fundamentals of fluid flow (Reynolds numbers and Prandtl numbers are all that good stuff) viscosity is generally never a big effect on flow rate in any circumstance. All it would do is create a bit more friction and increase your oil pressure. And there's the final thing. Everyday, everywhere, every engine more than adequately copes with oils over a massive viscosity range. This is because the engine starts from cold (when oil viscosity is very high) and gets hot (when oil is thin). If what you are saying was true, for ALL OILS, the top end should be starved of oil at start-up and that patently isn't the case.
Next, 'stuck rings are rare these days'...Lobe, do you hear that? You're one in a million! Do you want to say something here? Maybe now's the time to fess up that you've been abusing your mega-expensive BMW by putting the cheapest supermarket 20W50 in the sump, running 30,000 mile OCIs, running it on fuel that you reclaimed from rancid chip oil and then leaving it in a cave for ten years to rust up! Yes, it's all your fault! And that applies to all of you other windgers on BITOG with your Toyotas and your Hondas and your Audis and your Subarus that guzzle oil. Bloody attention seekers the lot of you! (sarcastic rant over)..
