Discussion
Errmmm... isn't it the case that the 'lore about not mixing synth and non-synth oils originated from the real problems experienced in the 2 stroke world when mineral and veg based oils were mixed? Once a bike engine, say, had been run on a castor-based premix, any use of a mineral lube would risk ring gumming etc unless the motor was stripped and washed first. I remember that rule being carried over when synth oils became the natural successors to the castor products but think that was more to do with tradition and habit than any real chemical properties of the new generation oils.
I started a thread elsewhere questioning the composition of Halfords own brand 'synthetics' and the company's own customer relations people confirmed on my asking that they are really refined from mineral bases not esters as used by the likes of Mobil or Silkolene but that they are compatible for topping up etc (and 'qualify' to be marketed as 'synthetic' on the basis of their lubricity. Provided the specs are similar, I can't see it being an issue but, if I'm wrong, maybe someone could explain exactly why in a non-dogmatic way. Please
I started a thread elsewhere questioning the composition of Halfords own brand 'synthetics' and the company's own customer relations people confirmed on my asking that they are really refined from mineral bases not esters as used by the likes of Mobil or Silkolene but that they are compatible for topping up etc (and 'qualify' to be marketed as 'synthetic' on the basis of their lubricity. Provided the specs are similar, I can't see it being an issue but, if I'm wrong, maybe someone could explain exactly why in a non-dogmatic way. Please
No it's not based solely on that. Similar things happened in the early 80s with mineral oils BTW causing sludge to be dislodged when owners switched to synthetics. Some oil types don't mix period. Mixing different grades of the same oil will cause the resultant mix to have a variable grade which may offer worse protection than the original. Mixing different makes may or may not have an effect.
Unless the chemistry is actually known it is difficult to predict what might happen. So the general advice is to keep to one type and not to mix, especially as the chemistry can change with budget brands. The situation is made worse due to the high stresses that TVRs put on the oil.
It's not black and white. It may or may not cause a problem but why take the risk? Oil is relatively cheap compared to other running costs.
Unless the chemistry is actually known it is difficult to predict what might happen. So the general advice is to keep to one type and not to mix, especially as the chemistry can change with budget brands. The situation is made worse due to the high stresses that TVRs put on the oil.
It's not black and white. It may or may not cause a problem but why take the risk? Oil is relatively cheap compared to other running costs.
So what exactly are these 'high stresses'? Last time I looked, the Rover based unit was a relatively lowly tuned, low revving unit, making low Brake Mean Effective Pressure. Its lubrication system may be marginal so far as the cam feed is concerned, and they might be prone to combustion blow-by and fuel contamination from over-fueling due to antiquated design, and over heating if the cooling isn't right but I see little that's inherently 'high stress' in the way that, say, a bike engine revving to 16k rpm might be? Agree good oil is better than poor and you generally get what you pay for 
Edited by pupp on Saturday 4th November 11:34
I was comparing the Rover V8 in a TVR compared to a Rangie or SD1. A 5L Rover V8 in a Griff is under more stress than a 3.5 with twin SUs in a SD1 for example.
Run mineral oil in a SD1 and it tends to be OK. Now run it in a TVR and it sludges big time and is unlikely to last the service interval. Main reason is that the TVRs are producing more power than the original Rover V8 cars and have more heat to get rid off and are generally driven harder. A synthetic is a better choice as it survives the conditions longer.
Is it as highly stressed as a bike engine doing 15000 rpm? No, but it certainly is compared to a 135 bhp Rover V8.
Run mineral oil in a SD1 and it tends to be OK. Now run it in a TVR and it sludges big time and is unlikely to last the service interval. Main reason is that the TVRs are producing more power than the original Rover V8 cars and have more heat to get rid off and are generally driven harder. A synthetic is a better choice as it survives the conditions longer.
Is it as highly stressed as a bike engine doing 15000 rpm? No, but it certainly is compared to a 135 bhp Rover V8.
Edited by shpub on Saturday 4th November 17:25
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