Had the first oil change on my 07 MX-5 done a few days ago at an oil change place (I won't go to the local dealer unless absolutely necessary) and had them use synthetic oil. I noticed later that the receipt indicated they used 5-30 synthetic instead of the 5-20 called for in the owner's manual. When I called them they said that they didn't have 5-20 oil and that it shouldn't be a problem. Seems to me that the oil pressure gauge is reading a bit higher than before, but before I go butt heads with these butt heads, is it ok to use 5-30?

 

Personally i would use a 10-30 oil. 5-20 is asking for trouble IMO. The weight of oil you use is largely dependent on the climate you are in. Hotter climates/lots of stop and go driving in heavy traffic need thicker oil. The 5-20 oil might be fine when fresh but the longer it is in the engine the lower the viscosity. So that 5-20 will be the viscosity of water before long and if you miss an oil change the bearings could get toasted,as well as the piston skirts. Pistons skirts are protected against wear from the cylinder walls by a thin film of oil between the piston and the cylinder wall. Bearings are the same scenario. Lots of manufacturers are recommending thinner oil but my personal belief is that is more for emissions than for engine longevity. The weight of the oil in the crankcase does effect compression/performance/emissions.

 

Thanks for the feedback. I'm old enough that I remember when 10-30 or 10-40 were pretty much the only choices, but 5-20 is the only oil Mazda recommends for the 07 Miata. The 5-30 that's in there now definitely makes a difference, and not in a good way; the oil pressure gauge is much more sluggish and the car feels tight above 5000rpm. If the service department at my local Mazda dealership would answer their phone or return a voice message, maybe they could shed some light on the matter.