RWD 19xx-1984 DeVille and Fleetwood, 1985-1996 Fleetwood and Brougham ForumForum for discussions regarding the rear wheel drive Pre80s-1984 DeVille and Fleetwood, 1985-1996 Fleetwood/Brougham.
I'm currently running 10W40 in my 76 DeVille and would like to know if it would be ok to use the same oil in my 89 Fleetwood Brougham's 307? The shop manual for my 76 says 10W40's cool in operating temperatures between 40*-100* so would it be cool to put this oil in my Fleetwood?
Would it hurt it? Most likely not, I wouldn't give it too much thought if it was mine. GM did abandon 10W40's and 20W50's due to additives causing increased frequency of sticking rings, but that was on old SE or SF motor oil, we have come a long way since then. Probably just fine. For sure, you wont hurt it.
You can try synthetic (real, like Mobil EP or Amsoil 30W(10W30) ), it burns slower and might slow it down, but can be a bit pricey. Being most synthetics aren't synthetic these days (thank you Supreme Court for letting the industry lie to us!). So only a few are, price typically dictates...
Thicker oil, like going into summer months, a straight 30 might be better and last longer, the additives burn first, so having more pure oil is better long term. But the pocket book has to keep up! Is it cost effective is the question, might be cheaper to just keep dumping in regular oil (My 455 Pontiac was up to 22 quarts for 1200 miles when I took it down to fix....)
Don't believe anyone. Do a google search on "motor oil bible", and there will be hundreds of sites from which you can download the approximately 177 page document (free). Then go to 5 or 6 oil manufacturers sites and look at the product data sheets for their various weight and types of oils. Be your own expert. NODIH is correct about why 10w40 was stopped by GM. However the chemicals used to extend it to 40 are still in it and leave deposits even though it is better these days. Dino oil has trouble being extended past a 3 to 1 ratio (the two numbers). With synthetics that is not a problem. Since these engines are apparently old, I would not use a class 4 (PAO) synthetic since that shrinks seals. Thats why the higher grade synthetics mix in the more expensive ester (class 5) which expands seals. When I am concerned with old seals shrinking, I use a class 3 synthetic which is dino oil with the molecules rearranged (This was a classic supreme court case--Mobil sued Castrol for calling it synthetic, Castrol won). I use Rotella 5w40 synthetic, a tough oil. If I am not concerned about seals I use Mobil1, 0w40 (mix of PAO's and esters with a lot of friction modifiers). My engines sing praises with this oil. The low first number is good for instant circulation on start up, which is when about 75% of wear occurs. I need the 40 because I am in a very hot climate plus my engines are high mileage (larger tolerances). Both of these oils have a high "total base number" (alkalinity,detergency). Because of this, I do an early oil change the first use (2000 miles), because it cleans out old gunk. After that, It's your educated judgment. I don't believe in thin engine flushes. There are a lot of experts out there--be your own. Warning, a lot of 5w30 and 10w30 synthetics do cause minor seal leaks. Mobil1 also makes a very "tough" 10w40 high mileage synthetic, but it doesn't circulate fast enough for me on cold starts--I hate hearing valves. ALWAYS use an extended performance oil filter. Oils can go a long way in a clean engine with good filters. A high (TBN) total base number is important in that case (at least a 10).
