I was first introduced to Synthetic Motors oils back in the 1980. I purchased a Mazda RX7 with the new Wankel engine. After a short break-in period, I put Mobil in its crankcase. It used about one quart of oil every 6,000 miles. At 60,000 miles, I converted back to regular multi-grade heavy-duty oil. In less than 3,000 miles, it stopped using any oil. I was told that I had not broken the engine in before going to the synthetic oil. Since then I have always use synthetic oils in my American cars with great success.
When I suggested using it in my beloved 1960 Austin Healey 3000, BT7, I was told by many that it would cause the vehicle to leak all over the place. But it was doing that any way.
Eventually I had the motor rebuilt. I drove it the recommended break in period. Looking for an oil that would work well in the desert heat, I chose Castrol 5W50. I did not read the fine print to see that it was a synthetic oil.
In "talking” in person and on the Internet to some far more knowledgeable than, I was told that it would cause oil leaks from the engine. I thought that was strange, since I had not had any so far. So using all my engineering skills, I contacted the technical staff of some of the oil companies such as Castrol and Mobil.
To the claim that synthetic motor oils will cause an engine to leak as opposed to regular oil the response was as follows:
It is a very common myth that switching from conventional oils to synthetics would cause seal leakage. This is due, in part, to the fact that when synthetic motor oils were first introduced in the automotive market (70’s and 80’s) there was experience of leakage in some vehicles. Synthetic based oils and oil formulations have evolved since these early days. Oil manufacturers learned from this early on and reformulated their synthetic oil to contain the proper additive package, which actually helps condition seals and gaskets to maintain their elasticity while- also preserving proper seal swell. Today in modern ILSAC GF-5 and API SN motor oils there is a requirement to ensure seal compatibility.
Another factor to consider about synthetics and seal leakage pertains to the additives in synthetic oils. The additives that give a synthetic oil its detergency properties tend to clean out the sludge and deposits left behind by petroleum or mineral oils and, once these deposits are gone, the gaps around seals and gaskets become open and oil could begin to leak from these gaps. It is likely that the additives in synthetic oils will begin to lubricate the seals causing them to become more flexible and seal those gaps over time. However, there is no way of knowing how long it may take this to occur or if the seals are already too far gone to be salvaged and replacement is necessary.
For your 1960 Austin Healey, we would not expect that our SW-50 synthetic grade of oil to cause any oil leakage issues due to incompatibility with engine seals vs. conventional oils. Elastomer compatibility is part of the current oil category and tested to the highest industry standards to ensure non-aggressiveness to seals. Any break in instructions should be followed but the use of synthetic motor oils from the start should not be an issue.
Castrol EDGE SW-50 (Black bottle) is suitable for normal street vehicles. It meets API SN specifications and is formulated with additional zinc anti-wear additives. For those wanting a more high performance lubricant, use the Castro! EDGE Super car product. This is approved for use in vehicles where the manufacturer requires a product that meets Ford WSS- 2C921- C. These specifications are for the Ford GT Supercar but can also be used for streetcars.
Summary: synthetic oils can be used in classic cars IF the gaskets/seals are in good condition and do not need replacement. It will not promote leakage unless the seals are defective.
Again, there was much discussion regarding transmission fluids. The Austin-Healey manual recommends standard 30W oil with no additives. I had always used this in my Austin-Healey, even though the Triumph and MGs recommended the use of gear lube. This summer I was having a multitude of problems with the over-drive. It took a long time to shift and then it was doubtful if it would hold. Even at speed, it would drop out. So based on the recommendation of some -the far right conservatives - I tried an 80W-90W gear lube.
The recommendation was as follows. There is a difference between the 30W oil and 90W gear oil. This major difference is in the thickness of the oil. A 90W gear oil performs like a 50W oil. It sounds like in the higher temperature climate that you live in; a thicker viscosity oil is needed.
If you would like to read a rigorous analysis and grading of motor oils, I suggest reading 540 RAT Tech Facts. NOT Myths. This analysis is bound to put you to sleep faster than counting sheep. It does compare the various oils, including Synthetics, with a clear grading system. The report is available on the Internet.
Here is- one their bottom-line recommendations I found interesting.
BOTTOM LINE: By the time a normal, daily-driven street vehicle reaches 5,000 miles on its conventional or synthetic oil, the oil is dark, dirty and contaminated; it is becoming more and more diluted as time goes on. It has been suffering some thermal breakdown deterioration, and if it uses viscosity modifiers/improvers, it is nearing the point where those viscosity modifiers/improvers will be breaking down enough to start affecting the oils viscosity.
Therefore, by that point any motor oil is in definite need of changing. And by changing it then, you will prevent any concerns about sludge formation. People who go much longer than a 5,000-mile oil change interval, just don’t understand the numerous technical reasons why that is NOT a good idea, even if they use very expensive premium synthetic oils marketed as extended change interval oil. But, now they know, so they can make a more educated decision about the oil change interval that is best for their engine
So draw your own conclusions as to the best lubricant to use for your vehicle. I wish you the best of luck on your choice.