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MG MGB Technical - An oil level question

Throughout the years I would check the oil when the engine was hot and add as needed. On trips that meant at gas stations after gassing up, etc.

Now I've noticed if I check the oil with the engine hot/warm the level is right at the top mark. If I check the engine cold the next morning the level is now past the top mark and at the M of Max on the dipstick.

If I let out some oil when the engine is cold and get it to the top mark I see that the oil level when the engine is hot is now down towards the lower mark.

Is this normal? Have I not noticed this all these years?

What should I use as a measure to get the oil to the right level?

For an accurate measurement you should wait 5-10 minutes after a run before you check.

It is most accurate to check in the morning before startup because the sump now has collected all of the oil.
Steven Rechter

I have to say that '5-10 minutes after a run' and 'in the morning before startup' cannot both be 'accurate'.

Oil can take some time to drain back to the sump after switch off, some manufacturers recommend checking cold and others hot just a few minutes after switch-off. My glovebox handbook doesn't say for checking, but for refilling says to check a few minutes after switching off. But it's not that critical and is why there is a Min and a Max mark i.e. you maintain it between the two. You wouldn't want it either above Max or below Min when hot.
Paul Hunt

BL workshop manual says 'a few minutes'

Brian Shaw

The driver's handbook says "always check the oil before a long journey", which probably means checking it cold.

Checking it hot, a few minutes should be sufficient for hot oil to drain back to the sump.

After changing the oil and refilling, the cold oil would take a long time to drain back. Running the engine for a short while will warm the oil, helping it to drain, as well as filling the filter.

I always check it cold, apart from after an oil change.
Dave O'Neill 2

Thanks everyone! I drained a bit out today after the oil change to get it in the proper place on the dipstick.

If you have an inverted oil filter, could it be that the oil is leaking out into the sump, so that the level increases? This will happen with a faulty non return valve.


Tore, that's interesting because I never recall this happening in the past. But isn't the non-return valve inside the oil filter?

It is inverted. 1971

Yes. The quality of the cartridge oil filters, if that is what you have, is varying. I have converted my 63 MGB to cartridge filters, and have noticed big differences in how fast the oil pressure is coming up when starting from cold.

In some filters the valve is not working well at all, it takes several seconds to get oil pressure even if the car has only been standing for a day or so, because the filter is almost empty. With the Mann W 916/1 filter I am using now I get good oil pressure immediately after starting, even if the car has been standing for a couple of weeks.


I used to use K&N forever and then they changed the specs without telling any of us and many suffered no OP as a result of the inverted holder tube being blocked.

Someone who used to work in the industry insisted that Purolator PL20195 was the best available and I switched to that. Don't see much of a difference.

Mann oil filters seem to be the best for having a decent non return valve. W916 I seem to remember. It's also a Volvo item as well.
Allan Reeling

Thx! I'll look into it.

There is a Volvo filter that fits the MGB - 3517857-3, however that is shorter than the Mann W916/1, it's equivalent to the Mann 917/1. The longer one is preferable as it has a fighting chance of having more filtration area.
Paul Hunt

I'll look for a cross index number as that Mann number only comes up for the UK. For years I was happy with K&N. Too bad they changed the specs.

Bear in mind the cross-index may well list everything that physically fits, regardless of quality. One list I have contains 126!
Paul Hunt

This thread was discussed between 07/10/2015 and 15/10/2015

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