Lubrication - Temperature critical 

The correct running temperature of the oil is perhaps even more important than the water/coolant temperatures - so let us consider how to control them. Mainly because it seems to be the most misunderstood of the whole process, and oil cooler fitment almost a reflex when over-heating occurs where a tuned engine of any type is concerned. And in many cases on standard production road cars for that matter. 
Almost since the appearance of the immortal Cooper S, fitting an oil cooler has been the essential thing to do to any tuned Mini without any comprehension why. The original fitment was necessary because oil and bearing technology wasn’t too advanced. Asking an oil of yester-year to cope with lubricating an engine at racing speeds, and a gearbox pushed their performance to the edge of their all too narrow capability, so extreme control over their working environment was essential for reliability. Hence oil cooler fitment. 
Giant steps forward in technology since then has benefited modern motor oils enormously. They’re far more capable than their ancestors, coping easily with broad temperature ranges without suffering total degradation. Correct temperatures are essential to disperse combustion products and other contaminants (such as water), and give maximum lubrication. Running the oil too cool is worse than running it a little hot. Short journeys cause low temperatures, protracted periods of hard driving excessively high temperatures. Constantly running the temperature outside the right temperature range dictates oil changes are needed far more frequently, and many miles sooner than normal. Generally by two thirds - and that’s no exaggeration. Incidentally - the short journeys syndrome is responsible for that milky substance that accumulates in rocker covers and breather pipes - it's condensation that has emulsified the oil because the engine/oil isn't getting hot enough to burn it off. 
Optimum performance, for both economy and power, is achieved at temperatures around 100-107 degrees C (210-225 degrees F). Up to 120 degrees C (250 degrees F) is OK if a decent oil is used, but getting a little warm. Some engine builders believe oil should be kept around 80 degrees C (175 degrees F) to keep things ‘safe’. This simply is not high enough for best performance. The biggest problem is actually measuring the running temperature in a Mini. You need to assess the mean oil temperature measured in the gearbox (sump temperature). Extremely difficult to do considering there’s no ‘natural’ position to mount the sensor. As a consequence, many folk fit an adapter in the transfer pipe from the block to the filter head or oil cooler. Unfortunately this gives a much lower – therefore false – reading, rendering fitment near useless, and wasted finances. However, as always, Mini Spares to the rescue. Much arm-twisting persuaded one of their suppliers in the relevant field to produce a couple of adaptors that screw into the sump-plug hole, replacing the drain plug completely. These adaptors then allow fitment of a couple of different types of capillary-type temperature gauges to be used. After all - the electric ones generally available are no more than 'guess-ometers', so are pretty useless. If you want to know what the temperature is you want an exact reading, not a near miss. 
So, what’s it got to do with engine temperature/cooling? The oil not only lubricates engine components; it also cools them by carrying heat away from them just the way water/coolant does in the cooling system. Much of the oil cooling is done by airflow across the alloy gearbox casing, some by the cooling system carrying heat away from the crankcase/engine block. Really good oil will do this more effectively than others, reducing general running temperatures without needing more cooling capacity, giving the cooling system an easier life. As it degrades, it becomes less thermally efficient. 
Most modern brand name oils are more than up to the job of dealing with a fairly broad temperature spectrum. Synthetic oils are far more tolerant of hot running without degrading. Some specialist oils work outstandingly in Minis. Few of them require coolers except in motorsport disciplines. Coolers work by air flowing through them, so one sited out of any reasonable air stream is worthless. Oil coolers that reduce temperatures by less than 10 degrees C are a waste of money. Trying to get the best of both worlds by using an oil thermostat with a cooler isn’t the answer. The thermostats generally open around 74 to 80 degrees C (165 to 175 degrees F) – under the required temperature for maximum performance, a bigger waste of money. Not to mention the hassle of fitting it! 
Rule of thumb and years of experience push to the fore where recommendations are concerned. It’s also one of those areas where less knowledgeable/unscrupulous types will sell you what they’ve got and what they want to sell (oils and coolers) rather than what’s best. Successful engine tuners have their own solutions. For guidance, there are some well-established ground rules. 
Road cars with engines producing up to 100bhp at the crank don’t need coolers unless they spend a good deal of their time in the maximum power bracket. If the car's a weekend warrior, fit a cooler (a 10 row one will be more than ample), but blank it off when on general duty. For a racer, make sure your oil temp is right. As with the weekend warrior theme, fit an oil cooler - it can always be blanked off with tape to achieve the desired running temperatures. Make sure it is in the airflow though - sticking it in the back corner of the engine bay is worthless. 
Some things to look out for when considering fitting an oil cooler. If a pre-engaged starter is used/fitted (integral starter solenoid) the oil cooler needs to be mounted closer to the alternator. Longer pipes than usual will therefore be needed. Up to 1992, the A-series used a block union size of 5/8-in UNF and filter head union size of 1/4-in NPT. From 1992 onwards, the union sizes on engines with the prefix number of '12A…' were changed to 11/16-in UNF on both block and filter head. The filter head can be recognised by the casting number LPX10027 on it. These latter units had an absolutely disastrous 'O'-ringed fitting that was supposed to hold the transfer pipe into the block - it didn't. The result was many upset late model Mini owners when their engine spewed oil out al over the place. Not to mention the damage it did when on the motorway before it was noticed! Consequently Mini Spares had a special steel-braided replacement pipe made with proper screw-in fittings. 
Use good quality oil. Valvoline 20/50 Racing mineral oil is very good in Minis at a very reasonable price. Torco 20/50 mineral oil is exceptional, 5/50 synthetic better, 20/50 absolutely brilliant - but all a bit more expensive. Most established brand named multi-grade oils are OK. Torco’s whole range of oils are the best at heat transfer I’ve ever come across, Red Line a close second but they only do full synthetic racing oils so are relatively expensive. Mobil 1 was the first real fully synthetic oil, but I've always found it poor at carrying heat away from major components (like crank and cam). I've had the same reports from other users in various fields of motorsport used in a wide variety of engines. Simply going from Mobil 1 that I used for a couple of years, necessitating a very large oil cooler to be used, to Torco 5/50 I saw such a huge drop in oil temperature I could run with NO cooler, and still had 10 degrees better temperature. And that was in a 1430cc, 150bhp, LSD equipped tarmac racer! 
If the car is used for mostly short journeys (under 20 minutes) or sporadic bouts of aggressive use, change the oil at least twice as often if not more when using a middle-of-the-road oil. 
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