I have been using conventional lower-end gear oil in my Tohatsu 9.8 HP 4-stroke outboard on my N22 MOUSTACHES. I was given a brand-new bottle of SYNTHETIC gear lube oil and, certainly, want to use it but …
Next year, I’ll drain all of the old CONVENTIONAL lower-end oil out, leaving a bit still left in the engine. Can I just go ahead and re-fill it with the SYNTHETIC ?? In short, can you “mix” conventional oil and synthetic, even tho there will only be the dregs of the conventional oil left ??
Any thoughts ?? More common is the question as it relates to “engine” (crankcase) oil - what happens when you decide to switch to synthetic in your crankcase ??
I am not familiar with Tohatus but Honda recommends 10W30 SG, SH or SJ oil. They are specific on the viscosity which is important. The amount that remains after a drain is not sufficient to make any difference, however if you use Shell for example I would stick with Shell when changing from dino to syn.
Mixing brands when you top up can have unintended consequences, less so when you change oil.
Most oil manufacturers state that mixing synthetic and mineral oil will not result in bad consequences, as in a pinch…oil is oil. What I would say though is that synthetic oil is a premium product, and you would be nullifying it’s advantages by mixing it with conventional oil. Far superior to fully drain the mineral oil and replace with synthetic. You can guarantee getting the dregs out by changing the oil when it is hot.
I run synthetic oil in my motorcycle, and when I changed from conventional to synthetic the idle speed actually increased…it’s that slippery.
On my boat I run synthetic in just above everything, including the diesel and transmission. Oil, even synthetic oil, is cheap…compared to a rebuild.
Synthetic oil is supposed to increase oil change intervals, but I think it depends on the application. In a diesel engine, the oil also acts as a detergent, constantly cleaning the engine. Therefore I would change it as regularly as you do now. Perhaps in a modern petrol (sorry…gas) car you could get away with longer intervals, the manufacturers say you can. If 5L of cheap mineral oil costs $50 and the synthetic version costs $100, you are spending an extra $50 a season, which is 1/10 of a ‘boat unit’…hardly worth worrying.
I run around 30 - 40 hours MAX on my outboard in one sailing season and religiously change both oils after that.
One way or t’other, you have have taught me that after draining the LOWER END MINERAL oil as best as possible, leaving only the dregs in the gear case, it is OK to refill it with synthetic oil.
I still use 10W - 30W MINERAL in the crankcase (engine oil) and would only top it up, if needed, with the identical oil.
