Topic: It's winter, it's cold... Time for an oil discussion.

[Dunk]

Given the first real extended cold winter in a while I got to thinking about cold weather performance and oil. My CB750 does not run as nice, particularly idle less smooth, at temps under 20* or so. Less smooth idle, clutch rattle more prominent. I got to thinking the cold start leaving for work pulling the bike out of my garage which is always minimum 55* is a lot nicer than after it has sat outside all day at work and could be single digits or generally below freezing lately when I leave. Oil light has occasionally flickered on after a few minute warmup and keeping lower RPM short shifting then coming to a stop at idle, low adn rough, oil light flickers on for a moment once in a great while. Never once as warmed up as it can get in such temps.

I have used Rotella T6 5w40 for over a decade in my CB750s, year round. Never gave it much thought. Figured synthetic and 5w cold should be serviceable... But it's still relatively thick at low temps and in single digits can take 20-30 mins riding to get a reasonably good idle with mixture screws and throttle stop adjusted for a nice idle at freezing or higher temps. I can tell some of it after first starting is from cold oil, more load from the oil pump and I give it a few minutes warmup time in these temps before riding easy lower RPM for the first 15-20 mins to get some temp into it. Oil cap is always thick milky all winter, I periodically wipe it off but oil itself looks normal.

I looked up the oil weights recommended for the first time in a long time. 10w40 recommended for general all around use. Below freezing recommended 10w. Is there a better cold temp oil than 5w40 Rotella T6? I have assumed the 5w is fine for cold temps but maybe something else would be better. I use T6 10w30 is my car, which is a fancy new roller cam 302 with tight clearances. Would that be a better oil even with the higher cold temp viscosity? Or something else entirely?

[HondaMan]

Maybe this will help remove some of the 'mystery' of multi-weight oil: when I was in the oilfield we often had seminars with Petroleum Engineers to help all of us understand oils and fuels better, as we built equipment that had to operate from -50 degrees (F) to over 125 degrees (F) when we moved our fleets about to various districts for different jobs. One of the most important things I learned from one of those seminars was HOW oil becomes "multi-viscosity" because in its normal state, that is impossible.

It goes like this:
whatever the lower 'number' is for the oil, that is the base oil weight. Then viscosity enhancers are applied to improve its upper temperature performance, and its viscosity at the desired upper temperature is that upper weight number. So, for example, 10w40 oil is 10 weight oil with viscosity enhancers (which are long to very long polymers of one type or another) so that at that upper temperature it has viscosity similar to that upper number if it was running at that temperature.

But, here's the thing to always remember: as the oil ages, it does so by breaking those longer polymer chains down, which makes the upper "number" get smaller and smaller as the oil ages. So, if you started with 10w40 oil and you were at 50% of its rated life, it will act like 10w25 weight oil. As it gets older it will act more like 10w oil, whether hot or cold.

In the SOHC4 CB750 world, the oil pump cannot suck on thin oil very well: its clearances were not designed to do that. Instead, the oil tank was set above the pump to try to help provide "head" pressure into the pump when the oil is thin (hot) because the oil lines are quite long back to the tank: this reduces how well the pump can suck the oil from the tank. In addition, the type of pump it is (gear pump) works better as the viscosity thickens. It is easy to observe this: when the oil is cold and thick the oil PSI is always considerably higher than when the oil is hot and thinner. In this engine, 10w40 oil produces typically 30-35 PSI when hot on a hot (80 degrees or more) day, while using 20w50 oil will improve this to 45-60 PSI at normal engine temperatures. This is why roadracers always used 20w50 or 20w60 weight oils in this engine: with normal use in traffic, having a 10 number as the base weight will provide barely enough pressure at idle to keep the oil PSI light OFF. Using 20w40 or 20w50 oils will improve this, a lot, and will help preserve the crankshaft and rod bearings (aside from providing a higher top speed, if needed).



Let the arguments begin: the science is now stated, for clarity.