Since we have Salesrep, the "go to guy" for lubrication questions, I'm going to take advantage of the opportunity. What are the dangers, if any, of mixing different types of engine oils? By that I mean, different weights (say a quart of 15/30 into an engine full of 10/40, or straight 30, etc.), or, different service classes (diesel engine oils in a gas engine, or vice versa). What makes the difference between diesel and gas engine oils anyway? This is an area where my specific knowledge has always had a definite gap. Anybody else with wisdom to contribute is welcome to as well... Thanks.
Mixing the different viscosities. Does just that. Mixing 5-30 and say 15-40 will give you a viscosity of somewhere inbetween. You will also be mixing different additive packages and base oils which may not give you the optimum performance you need. HDEO (Heavy Duty Engine Oils) are typically for diesels and usually in the 15-40 or 5-40 weight. They have more detergency and soot handling control than their gas counterparts. That's a start. More queries?
So in other words, you could just as easily be creating a positive effect as a negative one--if what you had in the crankcase to begin with was less than the absolute optimum, you wouldn't be hurting anything by mixing it with something that was closer to optimum. Obviously, if you had the absolute best product in there to begin with you'd be degrading its performance to some degree, but not to any catastrophic degree. The mixing act itself is not a mortal sin. If that's an accurate summary, then this question (and I'm not planning on pushing the envelpoe, so to speak, but I'm still curious)--is there a threshold value of some sort where you'll definitely, (or at least *probably* enough to avoid the practice), be having an adverse effect on your engine? (Sorry, I know that's an awfully specific question to ask for a topic that covers such a wide range of possible applications.) This is actually what brought the topic to mind. I recall a conversation many years ago with a couple of diesel truck fleet mechanics. They advised me that it was a no-no to use oil meant for diesels in gasoline engines. If memory serves me correctly, they cited the higher detergency as the harmful factor. They said that it might be ok to use one quart, just before an oil change, because that would help clean the engine out, but to use it more than that would damage the piston rings. Was that sound advice? (And before any of my friends step forward to tell unkind stories about me, I'll admit it. I have pretty poor habits with regards to lubrication anyway. But I'm trying to get better...)
Digger. you are right on traget. The mixing of oils will not be fatal. That said I would not recommend it. Oil comes down to three basic things. Base stocks, additive packages, and blending techniques. Base stocks and add pkgs. interact with one another in different ways. By adding another oil you will change the basic properties of the oil. Mixing oil or changing brands may cause some temporary leakage if switching "up" as the better detergency package can clean out the sludge that may have existed. I would NOT run anything but an HDEO in a diesel engine. Genreally a better base stock. Designed for soot control and a superior deterncy package (calcium being the best). Running HDEO in a gas engine is okay.
Ok. That directly contradicts what those guys had told me. I wasn't sure it made sense at the time, so I'm glad I asked (finally). Thanks.
If it helps I have run nothing but DELO eather 15/40 or 20, 30 in all my cars and trucks sence I sarted driving back in 65. amd my farther before me. I have never had a motor fail due to an oil problem. 60 corvair, 63 Harley, 69 Harley, 69 Z/28, 66 Fairlane GTA, 67 LTD, 76 F250, Damm there are just to many to list.