Castrol GTX Classic 20W-50 - viable option?

This oil is readily available in my area as opposed to the Castrol 4T SG rated. It's also $22 for 5 quarts as opposed to $7.50ish per quart for the 4T if I order it. I did a search on this forum and didn't see any discussion but could have missed previous posts. It appears it addresses the lower amounts of zinc / phosphorus of more modern oils and in the description and it specifically mentions "flat tappet, push rod" engines. The warning about catalytic converters seems moot if we already use recommended oils with high levels. My bike is a '94 R1100RS.

Check the archive issue of the ON for March 2013. I did a test of a number of oils including Castrol 4T. Listed are ZDDP numbers for all oils. The info might be a bit dated as for one, BMW no longer has their oil supplied by Spectro.

https://www.dropbox.com/s/eiofrwjyhchue9i/03_2013 Owners News.pdf?dl=0

or try this page https://www.bmwmoa.org/page/owners_news_13

So much history here, thank you to all the old guard

Appreciate the link, fascinating work. I wasn't able to locate an actual ZDDP number for the Castrol Classic. They did state that there was 1400ppm zinc max. And the specs show very similar percentages of zinc and phosphorous. Looking at the article you sent, seems like 1400ppm zinc would put it near the top compared to the ones analyzed? I'm not saying this oil is better than anything of course. I only am looking at it from a recent BMW owner / maintainers perspective and realizing there is a rich history of the oil debate. My thought was this "classic" version of the Castrol wasn't around during the peak of the conversation and now that it is, it looks like they are back filling the thing they were removing as they reformulated because of environmental, legal, financial, etc. issues.

I'm a little confused...they just say zinc, not the combination of zinc and phosphorous. Matt Parkhouse, in his previous oil analyses, suggested that sum of zinc and phosphorous parts-per-million should be in the range of 2000. In my article, one other oil besides the two Spectro products was Valvoline VR1 had good numbers. Time and again on various forums, the Valvoline oil keeps coming up. I believe it's a readily found oil. It was reasonably priced years ago, so I suspect it's in the same ballpark as your Castrol.

In my article, I also thought that viscosity was a good reference point. To be a 20w50 oil, the viscosity index has to be within a certain range. And since oil use over time tends to break the oil down, thus lowering the viscosity index, having an oil that starts higher in the 20w50 range bodes well for long time use. They kind of goofed up my graph on page 72 of the article. The blue bars represent the measurement of viscosity while the red dots on the red line show what % within the viscosity range the oil fell. A 20w50 oil viscosity range is from 21.89 to 16.3. Of the mineral oils, only two had a viscosity closer to the top end of the range than the others...they came out to over 60% of the range, ie nearer the top of the range. Those were Castrol 4T and Valvoline VR1. To me that says they have a lot of viscosity to "lose" before they fell out of being a 20w50 oil.

Generally speaking, get the best oil you can afford and change it often!!

If you change it every 6K miles as recommended, about anything works. -50 of course.

It's been a long time since any German car was approved for non synthetic motor oil. None of them have ridiculously short change intervals, of course.