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MG MGB GT V8 Factory Originals Technical - Castrol Magnetec

I am doing a service and oil change later this week and am considering changing to Magnetec for the winter.
I normally use a 20/50 (Duckhams or similar premium oil). The car is a factory GTV8 and of course the handbook was written 29 years ago. The question is then is Magnetec, being a modern 10/40 oil, OK for the engine or should I stick with the 20/50?
It is getting cold here now and I reckon the 10/40 won't do any harm for the winter - am I right?

Postscript to the above: Sorry - nearly forgot....
I also want to flush the engine out but have never used flushing oil before.

How is it used? How much to put in? What sort of revs to use? How long for? etc etc

Advice please - thanks. Martin


FWIW Paul H. and I are both using "Magnetec for Older Engines" in our V8's. Works well, still would prefer Castrol GTX 20/50 if it was still available. I wouldn't use that Duckhams crap if you paid me. High oil consumption on my engine and mine is in great condition. You shouldn't have any problems with the Magnetec just make sure it's the one for older engines.

74 V8
Neil Cotty

Martin - you would be well advised not to use flushing oil - goodness knows what it would do to the hydraulic tappets. The best plan is to use a semi-synthetic and change it religiously every 3000 miles. You don't need to pay too much for it either.

I have owned many cars with Rover V8 engines; a Rover SD1, two Range Rovers, a Discovery, an RV8, my B GTV8 which I have owned from new, and a Morgan +8.

All have performed faultlessly on Duckhams Q20/50 with oil and filter change without fail every 4 months or 3k miles. Typically engines seem to last 150k to 200k between rebuilds; maintaining the right anti-freeze concentration is critical though.

Nigel Steward

Well thanks to all of you. Neil: I didn't know that there was another grade of Magnetec but have now got the one you recommend. So thanks for that.

Roger: thanks for yours also - my car has a slight ligthweight tick, particularly on start up, which tends to disappear when the engine is hot. Oil pressure has always run OK in running mode and tickover as recomended in the book (can't remember the exact figures). I never move the car out of the garage until the oil pressure comes up (and even wait until the temp moves from the 'cold' white area) and lately it has seemed that the oil pressure has seemed a little sluggish in coming up compared to a few months ago (of course this could be because the weather is colder). This was one of my reasons for thinking that a lighter oil may be better for it. My local guru runs a 350bhp TR8 autocross car with the same engine and suggested that the tick could be one of the hydraulic tappets, which I understand have a small vent. He suggested that the 'tick' was probably a blocked vent hole and that a flushing oil usually cured this. Now I don't know what to do for the best because I seem to be getting conflicting advice.

Nigel: Yeah - well, I'm inclined to agree with you but obviously Neil does not. I've run cars on Duckhams 20/50 for years with no problems and the only reason that I posted this thread was to see if anyone using more modern oils could recommend them as users.
It seems to me that something like the Magnetic makes sense, especially in the winter.



oil pressure has seemed a little sluggish in coming up compared to a few months ago (of course this could be because the weather is colder), more than likely as mine is same. I'm just about to try Magnetec.


Cheers Paul - thought so - maybe the 15/50 will cure that.

OK I'll give more info as to why I don't like Duckhams. I have found _personally, _IMHO, ok :)) that it doesn't respond well to high heat situations and degrades rapidly leaving me with lower oil pressures than say GTX 20/50 or Magnetec etc etc.

I come from Australia and I've also used Duckhams 20/50 in my MGA/MGBGT/MGCGT (it's frequently the only 20/50 in the local shop) and it's _always resulted in lower oil pressure than the Castrol and higher oil consumption. Sorry, but thats just my experience. If it works for you use it, as Nigel says change it religiously every 3000mls or less it's the frequency of change rather than the brand of oil thats most important. If I really could choose my oil I would use Silkolene. Great stuff but costs an arm and a leg and very hard to find, prob. not worth the extra $ unless you Vintage race. With the Magnetec I've had neglible oil consumption over the last 15,000mls changing every 2-3K miles.

I've covered 45,000mls in my V8 in the last two years and found GTX 20/50 to work very well (*sob* - it's gone), Duckhams degraded quickly leaving me with much lower hot oil pressure and high oil consumption, Magnetec seems fine, but is also a bit sluggish to come up in the Winter time but that to me is usual at 0 deg C. For example my *hot* summer idle is around 15psi with the Magnetic and around 10psi with the Duckhams (74CB Factory V8). Simply a modern oil vs an old formulation and these V8's put out serious heat.

FWIW one of the contributing factors to my burnt out starter motor/s is the fact I use an ignition cut out switch, crank my car over till I see pressure on the gauge, then start the car. Just my own personal paranoia really. lol

Neil Cotty

Lightweight ticks are not uncommon - could be a tappet which is not revolving and therefore beginning to wear - or a worn rocker shaft - or even some chain noise - whatever, regular oil change will help prevent it getting worse - and here provided you change at 3000m - the key is the detergent - go for the highest cleaning at the lowest price.


Like Neil I tried Duckhams 20W/50 as the only named brand easily available now that GTX has changed from 15W/50 to 15W/40, which gave noticeably lower hot idle oil pressure that I wasn't happy with. I dumped the Duckhams after just 100 miles as it didn't seem any better than the GTX, and unlike the GTX which stays pretty clear on the dipstick over 3000 miles the Duckhams went dirty almost immediately.
Paul Hunt

The Maganatec for older engines gives noticeably better results than either, although it is doubly expensive by being more per can AND the can only holds 4.5L instead of 5L. All of them gave/give similarly slow pressure rise times, winter and summer. Rather than flushing oil (which I take to mean to drain out the old oil and fill with this special stuff) I use a pre-change treatment of Forte from Clive Wheatley or whatever it is that Halfords has at every change. This noticeably thins the oil and comes out blacker than non-treated oil, whether that is just a sneaky chemical change I don't know, but inside my rocker covers it is 'brown tarnished' as described by RPI which indicates low mileage (not in my case) or higher mileage but well cared for.

75 V8 with marbles in several cylinders
Paul Hunt

Why is this BBS not allowing the posting of anything more than fits inside this text box even though it has a vertical scroll bar?
Paul Hunt


Is that why it didn't post anything at all for you ...
Paul Hunt

What is your thought on Mobil One oil.
The G

Mobil/1 is good for modern engines, I've used it in my sprint/hillclimb Montego trubo for over 3 years. However beware of grade and cost. I buy mine in France where it is 5/50 grade and costs c. 18 for 4 litres. Here 2 grades are available, IIRC neither are as wide, and its well over 30 for 4 litres. I don't use it in the V8 though - that has Castrol.

So Modil 1 is not good for old engines? I noticed they had 15/50 in Mobile one.
The G

I've used Mobile 1 can't say it made any diff compared to usual Valvoline 20/50, no increase in mpg). As Nigel pointed out regular changes are more important and Mobile 1 is expensive.

I intend to try Maganatec (15/40 for conventional engines) for the top end rattle on start up.


Well I can get 5qts of Mobil One for $17.50 usd vs $5.85 usd for regular mobil oile. So it is a little picey but not that bad. I usually use Plain mobil oil it is cheap and I change every three months (unless for some reason I manage 3,000 miles before the 3 month period).
The G

Martin ,i have a factory BGTV8 with 4.2 litre here in OZ & run Penrite HPR 30 [20-60 GRADE] it has slightly higher pressure than 20-50 but great for hot climates, you may have to warm it up a bit in U.K winters. Its saved my engine when head gasket leaked water into sump, & internals stay like new even after 30000 miles.Watch using oils like Mobil 1 as made for newer twin cam engines & not older designed engines like ROVER V8s & A & b SERIES, you may run the bearings. Steve
Stephen Foldhazy


From Demon Tweeks Cat , Mobile 1 15/50 is for 4L (say 1 Oil change) 47.48 or approx $75 Magnetec is half this price and standard 20/50 cheaper still. So could cost an additional 100 $150 a year to use Mobile 1 in UK.


Yikes! Peeking over here from MGB, I just learned Castrol had changed GTX! Went out to the garage and checked my latest and it's GTX high mileage blend, but 10-40. Aren't I the observant one? When did it change for the US?

If you remember any advertising about '30% better protection', it would be about then. It was for the UK, and I didn't notice either.
Paul Hunt

Paul, That commercial catch-phrase does ring a bell. Ah well, as technology and time march on . . .

Jay. I am still getting GTX 20-50 in my area. Recently, the local Checker store started offering it in one gallon jugs at $5.99 per gallon. Picked up two cases for normal oil changes and a case of the one quart bottles for topping up between oil changes. It may be that the 20-50 is not selling well in your area. In Arizona, it is regularly available. Hope that this trend continues. Les
Les Bengtson

Well thanks to all of you - sure opened a can of worms there apparently (not to mention a can of oil).

The new oil filter arrived last week along with the new flame traps (about time I changed those) and the Magnetec for older engines is still in the back of the car along with everything else. It's been such lousy weather (which is usual when I decide to do anything) that the job is not done yet. And actually the dipstick is pretty clear so really no panic.

When I think about it you know I don't really know why we worry so much about things like this - I ran a Rover P6 3500S for about three years and never gave it a thought and the engine is not really very different.

Maybe it's because it has an MG badge on the front and the noise is not so damped (you could hardly hear the engine in the Rover).

This thread was discussed between 10/12/2002 and 19/12/2002

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