Real oil vs fake oil


I run a Volvo Penta MD 70B in my boat. This old baby has a 30 liter sump. Oil changes are getting spendy. A person who shall remain nameless says we should switch to synthetic oil and change it less often. Another person is saying yes but if you change to fake oil you can not switch back to real oil. And some of the places we travel to don’t have synthetic oils. So is it true? You can’t switch back and forth from one to the other.


Early formulations of synthetic oils caused rubber seals to swell at a different rate than conventional oils. Back then, switching oil formulations could result in leaking past the seals. This is no longer true. You can switch back and forth as the situation warrants.

It’s true that synthetics are less prone to shear and sludge so that oftentimes you can extend the change interval when using them. One caveat- if your engine rings are worn such that you have a lot of combustion blow-by, the oil will become laden with particulates at a fast rate. This alone can mean you have to change your oil regardless of the remaining condition of the oil itself.

With that much oil needed, I’d probably start using an analysis method for determining change intervals. These ‘pay for’ services actually measure what’s in the oil and guage its overall condition taking the guesswork out of an oil change. You send in a small sample in the provided bottle and they send a complete analysis that has a wealth of information about not only the oil, but the running condition of the motor too. As one example, bearing wear rate can be quantified. When using a lot of expensive oil, this is a small price to pay for peace of mind and to avoid a much larger unnecessary expense.

No problems with today's oils.  You can switch back and forth at will. 

That said, I would wonder if it might be best to stick with the current schedule and oil.  While the synthetic oil itself will last longer, there is also the issue of moisture and other non-oil accumulations.  I think I would stick with the manufacturer's recomendations.  You might try contacting Volvo and ask them.


BTW, synthetic oil is not fake. It is manufactured, rather than simply refined straight from crude. The advantages are better oil weight distribution and less ‘impurities’. Also, the flow characteristics are better, and it takes longer for the oil to breakdown. Nowadays, tho, you can even mix dino oil with synthetic without disaster. Synthetic blend oils are just that.


Buy your 15w-40 Diesel oil in 5 gallon pails at Wally World to control costs and stick with what you KNOW works…

Here, have some synthetic milk, it’s better for you. Right.

By next season, diesel fuel may be over $4/gallon. Yachting is getting more expensive every day. No whining…


Agree. I don’t normally recommend synthetic oil for older engines that consume some oil. Synthetic oil is very stable and slippery, and needs only a thin film for proper lubrication. Unfortunately, an old, worn engine relies on a THICK FILM to seal the combustion gasses in. Synthetic oil often causes blowby in an older, worn engine.

If your engine is in near new condition, you could use Synthetic at about 2.5 to 3 times the cost. If I were you, I would buy normal mineral oil of the spec required and buy in bulk. That would be more economical. Synthetic oils are most effective in extreme (heat, cold, loads) conditions, none of which apply to your boat.


You could probably extend the oil change interval, bit it likely not worth it. Synthetics were first sold to fleet owners with their own shops. They could extend the interval and save by having a smaller staff. You can’t get that same savings, unless you’re Jay Leno.