Let's talk about engine oils.
Some say use synthetic oil in a Delta engine, others say use a semi-synthetic. A fully synthetic oil is said to be too thin for older engines, which have larger tolerances. A mineral oil is said to be too thick in the cold, and not good for turbos, well here's an opinion.
Good synthetic oils often use a combination of up to three different synthetic base fluids, including synthetic esters, polyalphaolefin (PAO) and alkylated aromatics. Regardless of the exact chemical makeup, synthetics share similarities. Whereas conventional oils contain molecules of varying sizes, the molecular structures in synthetics are consistent in mass and shape. This uniformity means those molecules create less friction as they collide, and less friction means less heat.
As a result of this control over chemistry, synthetics have a number of significant advantages over conventional oil. You already know that synthetics flow better in cold weather, making engine starts easier and offering better protection. In addition, synthetics resist viscosity changes at extremely high temperatures that can break down or evaporate molecules in conventional oil. When this kind of deterioration happens, metal-on-metal contact may occur, causing permanent damage to the engine.
Because synthetics are slicker than conventional oil, they allow for smoother, cooler engine operation. That means more horsepower and better mpg. But in some cases, that same super-slick consistency may exacerbate existing internal or external engine leaks, resulting in blow-by and burn-off. Keep in mind that this happens only in engines where leaks are already an issue.
Some pundits also claim that synthetics are so slippery that in brand-new engines, the lubricants prevent proper break-in of motor components. And then there's the indisputable fact that synthetics cost at least twice as much as conventional mineral oil. However, that increase in cost is offset by the longer intervals between oil changes. Instead of 3,000 miles (4,828 kilometers), you can push off oil changes for every 7,000 miles (11,265 kilometers) or so. Some experts say you need to drain synthetics only at 25,000-mile (40,234-kilometer) intervals.
The above was copied from a larger How Stuff Works article. On a seperate occasion I was told by an experienced car parts man, that synthetic oil is better for any engine, old or new. When I suggested the oil was too thin for old engines, he said what do you think ordinary oil gets like when it's hot? Well, thin is the answer. There is an issue with old oil seals leaking, but perhaps then it's time to renew them. But his main argument which applies to a Lancia Delta engine, was about the turbo. Mineral oil crystallizes when heated, the oil gets very hot in a hard working turbo, and the hard burnt oil crystals can cause damage to bearing surfaces.
This argument convinced me about the benefits of synthetic oil. I had the engine in my 16v rebuilt 5 years ago, this was done using a new crank and all new bottom end parts, I ran the car on a good semi-synthetic for 3 years. Trips to Italy were made starting with clean oil, and on return home the oil was black, and ready for another change. After my car parts talking to, I changed to full synthetic. The oil pressure remained the same, there were no leaks, and the engine used no oil on long trips, however on returning home the oil was as clean as when we left. This fact alone convinced me that for a small, hot, turbo engine synthetic oil is best.
Fully synthetic oils weren't freely available when The Lancia Delta was built, so the instruction book would recommend the best available at that time. However the advances in lubrication available to us today should be fully utilised.
My actual choice of synthetic oil is Mobil 1 motorsport formula 10w-60.
The heart of our Deltas
Lovely fresh oil
A 16v far away from home
First start up after a rebuild
WITH LONG TERM REPORT
The Dealer Collection Delta shown above joined our fleet in 2013. The car had been well used, and was well used by us too.
After receiving an oil pep talk I decided to use Mobil1 synthetic oil in this car. After nearly 150,000kms I figured the engine was due a rebuild anyway if the experiment should fail. So I cleaned the old semi-synth out of the engine, and filled with fully synthetic oil, Mobil1 Motorsport 10w-60.
2013 went by, and 2014 too. I changed the oil and filter, and refilled with synthetic oil, ready for summer 2015. That year we went to southern Germany. After 1000kms of fast motoring, the traffic got worse, and the hills got steeper, the weather was hot and the Delta boiled up on a steep, slow hill. I'd never seen the hot warning light before. The temp was off the gauge, but the oil temp was still normal, we pulled over for a while. The motor was hissing and steaming, but the oil pressure was fine, this happened twice on the journey.
I have no chemistry knowledge, I just remember what I'm told!
Like on a crusty frying pan, hot mineral oil wants to return to carbon.
A hot turbo has the same effect, cooking the mineral lube oil and forming carbon granules. These bits travel through the engine and damage bearings. This heating and crystalisation is what causes the oil to turn black.
After a while the Delta cooled down, and we proceeded on our journey. When we next stopped for juice I checked the oil level, it was perfect, none used and clean also.
Returning to the UK, our little car had a few probs, none of the problems was oil related.
The oil was still at the same level, and still clean. The motor still had good oil pressure, no leaks had occured, and no funny noises.
I think Halfords use Mobil 1 as a loss leader at their stores, it is expensive,
but seems to be cheaper than parts stores.
I don't think we should be afraid of using fully synthetic oil. Their molecular structure is designed for smooth, constant lubrication under all conditions. Mineral oil has a random structure, and is affected by temperature changes. The thin synthetic can flow easily at cold start up, and retains constant viscosity over all temperatures.
So my long term opinion is...synthetic oil is ok in a Delta!