Oils ain't Oils

Contributed by Brian Weston

Goodday Colin,

Thought you might be interested in this.

After the saga with Murray’s Delight, I went back to complete my project to fit an oil cooler to my trusty red Mk 3 Sprite. Neil had recently rebuilt the engine, although he did not regrind the crankshaft which had about 1 thou of wear (the car has done 140,000 miles since I bought it new in Nov 1964).

The head was also fitted with hardened valve seats and planed slightly, so it now has a little over 9:1 compression. Unleaded fuel burns hotter than leaded, the increase from 8.9 compression means a little more heat, and it presently has a 30% antifreeze mix for Canberra, so that is less cooling efficiency. I am sure that Neil also used the 185F deg thermostat rather than the 175F deg that I usually put in every summer and removed every winter. The only plus on the heat exchange ledger is the yellow plastic cooling fan that I fitted instead of the old four metal blades.

After 300miles I drained the run-in oil and filled with (Supermarket Brand X 20W-50). Surprise, the oil pressure slowly dropped until at a 4,000 rpm test point on a 32 deg C day with the engine fully hot, the oil pressure finally stabilised at 42 psi – coolant was 186F. The coolant was a little hotter than before noting than in its 40 plus years the trusty oil donk and radiator combination has rarely exceeded 190F even on the hottest day (but generally with water as the coolant and using leaded fuel), and the oil was quite a bit below the 55 psi that it used to stabilise at.

I conducted some detective work on oils. Brand X and Brand Y supposedly buy the cheapest base oil and then add additives (that is why you can buy them at $12.99 at Woolies). Penrite supposedly buys quality Caltex base oil then adds its bits. Mobil also reputedly builds up from quality base oil (that is why you cannot buy Penrite at Woolies and Mobil is rarely listed as a Woolies special). So I bought the newly released Penrite 20W-50 at $17.99 at Autobarn. Externally the Penrite 20W-50 specs suggest that it should perform identically to Brand X 20W-50.

I filled with Penrite 20W-50 not even bothering to change the oil filter and remove the last Brand X traces. As soon as I started driving the car it was clear I had an oil pressure that degraded minimally with heat. After heat soaking the engine at my 4,000 rpm test point on a 30C plus day the oil pressure stabilised at 47 psi – with coolant still at 186F.

So Colin - OILS DEFINITELY AINT OILS - and I can see why some of the tradespeople I spoke with talked with hushed voices and furtive glances.

Subsequently I have fitted the oil cooler which took another 0.5 litre of Penrite to fill I before I connected it up. First up, I lost about 3psi when cold (down from 66 psi to 63 psi). No doubt due to a pressure loss through the pipes and cooler before the oil registered on the pressure gauge pick up.

But so far I have experienced very little oil pressure degradation with heat. At my 4,000 rpm test point on a 28C day, it holds 58 psi, with the coolant at 188F. The coolant is definitely a little hotter all through the driving range - no doubt due to the oil cooler restricting and pre-heating the air flow onto the radiator.

For next summer, I will drain the antifreeze and replace with water and inhibitor. If that does not help hold the coolant temperature constant I will revert to my old practice of using a 175F thermostat in summer.

But the interesting bit was that the specs on the oil container seem to bear little resemblance to the performance of the oil in the engine.