Basically, I think I had inadvertently used an oil (Castrol GTX Magnatec 15W/40 - for conventional engines) which thoroughly cleaned the engine and created a lot of sludge. Then I disturbed the sludge in the oil transfer housing while changing the head gaskets and the end result was rumbling big ends.
Since then, I have researched the facts behind oil and found some conflicting accounts but generally learned a lot. One very good source was the discussion forum on TriumphStag.net. I have copied one very informative letter to save you searching for it. I have also included a link to the Vintage Triumph Register which has another interesting article.
For those who just want to know what oil I will be using, I intend to switch to Halfords own brand 15W/50 at £6.99 per 5 litres! ("Basic mineral motor oil. Suitable for most pre-1980 vehicles", API SJ, ACEA A2 B2)
Oil facts - The Vintage Triumph Register
Letter by James H. from the Discussion Forum:
I used to sell lubes for a living so here's what I know.
You need to distinguish between the oil's SAE viscosity versus its API quality. SAE (Society of Automotive Engineers) viscosity tells you how well the oil flows at test temperatures - 0 deg C for the 'w' or 'winter' part of the spec, and 100 deg C for the summer.
Viscosity starts at 0 and goes up to 60, and broadly speaking, the lower the number the thinner the oil. Generally you want an oil which is thin in cold temperatures and thick in high ones.
Theoretically then the "ideal" oil would thus be 0w/60, meaning it's as thin as can be made for cold temperatures and as thick as can be made for high ones. The greater the difference between those numbers, the more costly the oil to make, because you need several different base stocks to get there.
Most engine oils are made at refineries that have a lube leg fed by a lubricant-rich crude oil. The main drawback is that refineries essentially extract products in order of lightness. All crudes contain wax; wax is heavier than engine oil; so all mineral engine oils have wax still in them. When the engine's cold, the wax sets the oil solid, so when you fire up your engine it runs unlubricated for a few hundred revs until the wax breaks down. This is why most engine wear is thought to happen at startup from cold.
There are a number of ways refiners jazz up lube quality, ranging from adding hydrogen to buying an off-the-shelf viscosity improver, but *all* minerals without exception have this drawback.
Synthetics get round this in that they're made by recombining other refining byproducts and are thus wax-free. You can also tailor the viscosity to pretty much whatever you want. When I used to work for Mobil we recommended 25,000 mile changes for Mobil 1, but in the lab it actually lasted for well over 100,000 miles, and still stayed in grade. This was never publicly recommended because most engines dump a certain amount of fuel into the oil over 100,000 miles, which does it no good.
Semi-synthetics sound like they're 50% synthetic, but Esso Superlube at one time was only 3% synthetic by volume.
API quality is the other part of the spec. Another reply correctly describes Spark and Compression performance and how to understand it. The API tests check ability to stay in grade (eg will the oil still be a 20/50 after 10,000 miles or will it have deteriorated into a 30/40?), effectiveness at holding dirt in suspension (vs letting it stick to the metal), anti-foam properties, anti-wear properties, anti-rust properties, shear stability (mainly a truck spec), and so on. Also, in theory a 15w/40 will cook at a lower temperature than a 15w/50, but you can correct that with the additives used. So if they meet the same API spec a 15w/40 and a 15w/50 are much of a muchness.
There used to be a Euro spec - G followed by a number - but you don't see it too often these days. A lot of oils often carry various manufacturers' specs on them too. These are usually similar to the API specs but some reflect manufacturers' concerns. Eg in the late 80s Mercedes had a problem with older engines sludging up when detuned to run on 95 RON unleaded, so their spec focused on that.
Personally I suspect that putting anything as thin as a 0w, 5w, or 10w in a Stag engine is probably a bad idea. The engine was built when most oils were 20w, so the tolerances reflect that. You risk the oil not staying in certain parts of the engine because it's thin enough to escape, and in fact there's a distinct possibility that the oil will fall right out. You're probably OK with the odd top-up of a 10w; I can't see a 15w presenting a problem either. It's not that far away from a 20w.
As for specific brands, tough to say. I always had a low opinion of Castrol, because they had a habit of claiming something about their oils as though it wasn't true of anybody's else's. Eg they'd trumpet that GTX was "low in phosphorus" and thus "kind to catalytic converters", when in fact *all* oils were. They also claimed something silly about some strain of GTX having a "special" wax inhibitor when of course all of them do. GTX was always the Persil of engine oils - it worked but was either exactly the same as everything else, or not as good.
Shell were also a bunch of chancers - they used to promote a mineral oil at a synthetic price because it was "hydrocracked". In other words, their lube plant was rubbish so they had to add hydrogen to the stream to get a decent lube out of it! The scam there was that they sold it into car dealers at a high price with the promise that the dealers could sell it on at an ever higher one. Shell priced it very high on their forecourts which meant that their car dealer customers could, too.
Ford oil was always worth avoiding too. We used to sell oil to Ford and they just wanted their cars to be cheap to run, so they'd demand the cheapest multigrade possible and then alter the specs to say that Ford engine had to be run on that. At one point they were claiming that Cosworth Sierras needed to be run only on Ford's cheap and nasty 20w/50, which is barmy.
That said, none of these brands is actually bad. I always look for the cheapest 15w/50 I can find and had no trouble with it in a Stag engine. It stayed in mine and it worked. The other day I was looking at Sainsbury's Homebase 20w50 and even that stuff is not bad! - for a Stag or similarly older car. I wouldn't put it in a modern performance job.
The best piece of advice I'd offer is DON'T for Gawd's sake pay up for one of these ridiculous "vintage" oils. All API specs comprehend all previous ones, so if an oil's an SJ it will also pass SC. A "vintage" or "classic" oil is just a cheap and crappy oil made to an obsolete spec and marketed as though it's somehow special.
Longwinded reply due to slow day at work!!
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