Re: changing to mineral oil
Posted: 28 Aug 2013, 22:17
vanisveryrusty wrote:Should I use a decent 20/50 then ?
I ain't going to tell you what to do! <grin>
I wouldn't get hung up on "20w50" necessarily. That's the viscosity that cheap oils have long been, with Joe Average being (usually mistakenly) under the belief that it's the "right stuff". It rarely is. Have a look at the handbook for VW's original recommendations as a starting point.
The two viscosity numbers of a multigrade are the relative thickness at zero deg C and 100 deg C. They're relative to the viscosity of "straight" oil of those ratings - so a cold 20w50 is as thick as a cold 20 oil (fairly thin), but hot is as thick as a straight hot 50 (fairly thick).
A 10w40 is a chunk thinner cold - which helps a lot on cold starting - but is almost as thick hot. You want an oil that's thick enough to maintain oil pressure when the engine's hot, but not too thick, since that'll increase the pumping losses as you shove all that treacle around.
Roughly speaking, the thinner cold the better - with the caveat Martin mentioned earlier about draining down the tappets borne in mind.
Roughly speaking, the thinner hot the better - with the caveat about hot oil pressure borne in mind.
If you've got an engine with shagged seals, then a thicker hot oil MIGHT stay in better - but it won't be some miracle fix.
If you've got an engine with worn internals, then a thicker hot oil SHOULD keep that oil pressure light at bay at hot idle - but it won't be some miracle fix.
If you've got an engine held together with a thick layer of gloop after previous owners have failed miserably at servicing, then a cheaper oil without as many or effective detergents MIGHT help to keep that in place and prevent leaks developing - but I'm not convinced an engine held together with gloop is a great thing...
The base stock - mineral, semi-synth, synth - doesn't give any hard and fast clue as to quality, either - except as a very general three-tier rule of thumb. There's one heck of a lot of myths about them. There is no such thing as an engine that can only run on mineral. Synth doesn't eat seals. Synth doesn't cause engines to leak - but the detergents in better quality oils MIGHT remove the gloop that's keeping it behind shagged seals. All three can be mixed.
The one thing that is broadly true is that - for a given viscosity - a mineral will hold up less well once the oil temp rises much above that 100degC viscosity rating temp than a full synth, and a semi will be somewhere in the middle. Some engines are recommended only to use semi or full, some full-only - particularly turbos, where oil carbonising in the bearings of the turbo can be a big problem, because of that better resistance to extreme heat. It seems to be generally regarded that our WBXs have a bit of an issue with oil overheating. The oil-water cooler in the 2.1s isn't a great way of shedding the heat, since the water'll be damned hot at the same time the oil is (but it's a good way of warming the oil up quickly from a cold start - and definitely better than nothing) whilst various well regarded engine builders in the US give extended warranties if there's an additional external cooler fitted.
I've been running my ~200k mile DJ on a 10w40 or thereabouts synth or semi-synth for ~40k miles (almost all of that in 18mo) in ambients from below zero to 45degC. Mostly it's been decent brands, sometimes it's been whatever we could get locally without having our wallets raped. It's a heavy hightop Westy, and was heavily loaded whilst we were on the road in it. I don't drive it particularly gently. I don't change the oil every couple of thousand miles. It doesn't leak oil. It doesn't burn oil. I get a little bit of low oil pressure at very hot idle. If it was worse, then I'd increase the hot rating to a 50. After several weeks sitting around unused, I'm getting a little bit of tappet rattle from cold start. If it was worse, then I'd increase the cold rating to 15 or 20. I wouldn't go near oil additives with somebody else's bargepole.
Give this information as much weight as you want, taken alongside whatever else you find elsewhere. Do what you want. But remember that engine oil is the main thing that stops your engine dying. Saving a couple of quid on a vehicle you want to keep is total short-termism, imho.
OpieOils.co.uk is a good source for a wide range of oil, and to see what's on the market. They're not the only source, or necessarily the cheapest, or even the widest range. The net is wide.
