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Our old magnum isn't on the road as it's awaiting a restoration, but we start it occasionally just to turn the engine over and let it run.

Recently though, the oil light came on when the car had come up to temp. What things should I investigate first?

As a very first port of call, I thought I should change the oil as the stuff in it has been in there for some time and I have no idea what grade it is either. I don't have my manual handy at the moment, so could someone also tell me what grade of oil is best for the 1800 engine?

Go for a quality 20/50 oil. If you let the engine idle to warm it up the water temp gauge may show normal but the oil temp may have risen way above normal operating temp and get extremely thin. This is a not an uncommon problem with the slant 4 especially with old oil, a low oil level and reasonable miles on the clock.

If the oil does reach this 'shear temperature' enough times it's performance drops off dramatically at lower temperatures too and the critical shear temp also drops way down as well. This results in lowering oil pressures as time goes on.

So, to cut a long story short, change the oil for a good one, only let it idle at operating water temp for a short time and if possible, take it for a drive to warm it up and get cooling air around the sump at the same time. A large capacity oil filter also helps with the cooling a bit as well.

If your car is used in traffic a lot consider the addition of an oil cooler as a worthwhile safety precaution.

Thanks. That's really helpful. I think it has some 10w40 in it at the moment looking at the bottles on the shelf next to it. It would have been put in there when we were getting it started again, with the intention of changing it once we knew it ran but that hasn't happened unfortunately.

Unfortunately going for a drive isn't an option at the moment. Too much rust and seized brakes, (hence no chance of an mot etc!). It's just waiting until I get the time to do the work on it. So idling and moving it out of the garage is the only thing we can do with it at the moment. But I shan't do it for long periods if the oil change fixes the current issue.

One more question for now - synthetic or mineral or part synthetic? I've never been really sure which is best for which purpose.

I believe you want mineral oil in an engine of that age, it's not really suited to synthetic or semi-synthetic oils. Also try to run it for a reasonable time - five minutes or so isn't necessarily going to be enough for it all to warm through properly.

I have a similar issue with my oil pressure warning light, but as the car also has a gauge I can see that the pressure is actually fine, so I'm assuming it's the pressure switch and intend to swap it before next summer. Might be worth checking the pressure switch (though all you could do is swap it) but also check the connection is not loose.

The problem with synthetics or semi synthetics is that a 20/50 is not available. If one was available it will have better high temperature longevity and therefore quite suited to the slant 4 in that respect. There is an expensive 0/50 racing synthetic available which in theory should be OK but I'll stick to good old mineral 20/50.

HI,pretty much as everyone has said a thicker grade oil will help the oil pressure in an older engine, but i'd check the sender as well. I've previously run a 15w50 from Castrol in my Firenza and it's been fine but the engine doesn't have many miles on it. Concerning synthetic 20w50,I have run that in my classic mini engines using Amsoil synthetic which has been discontinued sadly as it was good stuff. The only other car 20w50 fully synthetic i know of is made by Redline but it's pricey.
Regards,Dave.

Thanks all. I'll change the oil and take it from there. The sender connection is fine (doesn't mean the sender itself works though of course). Hopefully it's just a combination of pretty old oil of the wrong grade that has only ever been idled (and potentially run too hot if idled too long). Mileage wise, the engines only done about 70k in its whole life (dads owned it from new so we know that's genuine).

(Hopefully will get the garage tidied properly soon, then the engine and box can come out ready to crack on with the welding. Until now I hadn't considered doing much with the engine as it seemed pretty sound mechanically, but I wonder if I should do anything with it whilst it is out? Might start a new thread on that though)

I had bookmarked a while ago a 20/50 semi-synthetic by Millers on ebay, haven't tried it, but it will be my oil of choice come the next oil and filter change. Currently using 'Comma' brand 20W/50 oil.

http://www.ebay.co.uk/itm/MILLERS-OILS- ... 0338474004?

Ok had a bit of time today.

I got some 20w50 mineral from halfords and a new oil filter. First thing I was going to do was just warm the oil up thwt was in there by running for five minutes. I didn't think this would do any harm as the oil light was previously coming on when hot. Today though, it wouldn't go out at all regardless of revs even when cold so I shut it off and drained the mostly cold oil.

Refilled with the fresh 20w50 and same thing. Oil light on all of the time. I pulled the oil pressure switch, but there's not much you can do with it so I gave it a clean up, poked a soft loop of cable in there (I guess there's a diaphragm in there that might be stuck) and replaced it. Started the car and light came on with ignition and off when running at idle like normal. A few minutes later though, it flickered back on at idle, but would go off with higher revs.

Is this sounding like a dodgy switch? I'm going to order a new one and see. Apart from that worn oil pump?

Is there an off the shelf part that has two ports that I can screw in there that would enable me to fit a pressure gauge?

(Oh, and I curse the monkey that installed the last pressure switch, they had pretty much rounded the whole thing off so it was nearly impossible to get a spanner on there!)

Yep, sounds like a worn oil pump to me... Had this problem at Billing. Had pressure when I pulled up at the gate. Stopped engine for a couple of mins to get paperwork, started engine and behold, no pressure.

Leaving it to cool and adding stabiliser to thicken made no difference... Good job I carried a spare.

In fairness, the amount of time the oil light had been staying on after starting had been slowly increasing over three months or so (Darth is my daily runner) so I should have half expected it to give out at some point.

Dan