Re: Synthetic oil in 72' omc?
nodumdrivers said:
Prob been talked about before, but didnt see anything. The 1972 omc 155hp v6
The manual says SAE 30 oil.
oil information/recomendations from a 1972 manual is completely out of date. You cannot compare monograde versus multigrade oils from that era with those of today.
I'll try to explain best I can, from what I've read from reliable sources more or less, back in that era the recommendation of monograde oil (sae 30) over a mutigrade oil was probably because oil refining and processing methods weren't that great nor were oil additives. Engines that ran hot and for a long time on the same oil would break it down and that would lead to varnish, sludge, viscosity breakdown, wear, and engine failure. The multiviscosity oils like 10w-30 and 10w-40 were more prone to oxidation and viscosity breakdown because the additives in it were susceptible to breakdown under those conditions. Without going into great detail, additive key words off top of my head related to this are pour point depressents, viscosity index improvers, dispersants, antioxidants, anti-wear agents. With multiviscosity oils like 10w30, the additives in it are what gave it it's low temperature, and high temp, viscosity characteristics & performance but are also what would break down under unfavorable conditions and lead to oil failure. 10w-40 oils were more prone than 10w-30 due to breakdown, since the spread between the operating viscosity (30 vs 40) to the cold temp W (winter) performace viscosity required more additives. From this info it's comon sense that you wouldn't recommend a multiviscosity oil in a marine motor which you know runs under a heavy constant load all the time and runs in unfavorable conditions. Combine this with knowing a carburetored engine will most likely dilute the oil with fuel lowering the oil's viscosity (remembering multi-visc. oils will shear more easily) the best choice is a monograde oil without a lot of additives that can break down. From there it's a matter of environmental operating temperature as to whether you go with a mono 30 or mono 40, or 50. The higher viscosities will not flow when cold and depending on how cold ambient temp is on a cold start you may be doing more harm than good going with higher viscosities like SAE 50. If you can find castrol monograde oils, read the back of the bottle. I've seen SAE 30 say do not use below 32F and SAE 40 not below 60F. An SAE 30 fits the requirements of a marine motor best overall when you consider people use boats in temps < 60F. An SAE 40 would be a better choice if you're in a warm climate and would never see start up conditions less than ~ 60F. From there you would assess how hot the oil will get under operating conditions and select the proper operating viscosity (sae 30, 40, or 50...) Generally, the hotter it runs the higher the viscosity you'll want because the oil will thin as it gets hotter, and you need a minimum viscosity for a given operating temp so the oil will support and lubricate moving components. You should also take into consideration expected fuel dilution, and that generally requires selecting a higher visc. oil.
In addition to the additives, base oil processing and finishing of the 70's left a lot of impurities compared to today's standards. Things like wax and sulfur would cause the oil to break down and lubrication to fail. And blending base oils of different viscosities are how you first make a multigrade, so again from a 1970's standpoint it's not hard to see the rationale of starting with a good monograde base oil for a durable oil.
If you consider oils of today, base oil manufacturing/processing/finishing is much much better along with additive technology. No sulfur, wax, and other impurities in the base oil and the additives are less prone to breaking down. Some oil blenders get away with just blending high quality base oils with little to no additives that modify viscosity to get a desired miltigrade oil. Search on Group III base oil, versus traditional group I and II oils. hope that helps.
